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![Ken Helias](/assets/img/avatar_default.png)
All other add functions for lists have the new item as first argument and the position where it is added as second argument. This was changed for no good reason in this function and makes using it unnecessary confusing. The name was changed to hlist_add_behind() to cause unconverted code to generate a compile error instead of using the wrong parameter order. [akpm@linux-foundation.org: coding-style fixes] Signed-off-by: Ken Helias <kenhelias@firemail.de> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com> [intel driver bits] Cc: Hugh Dickins <hughd@google.com> Cc: Christoph Hellwig <hch@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1031 lines
38 KiB
Text
1031 lines
38 KiB
Text
Please note that the "What is RCU?" LWN series is an excellent place
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to start learning about RCU:
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1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
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2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
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3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
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4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
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What is RCU?
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RCU is a synchronization mechanism that was added to the Linux kernel
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during the 2.5 development effort that is optimized for read-mostly
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situations. Although RCU is actually quite simple once you understand it,
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getting there can sometimes be a challenge. Part of the problem is that
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most of the past descriptions of RCU have been written with the mistaken
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assumption that there is "one true way" to describe RCU. Instead,
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the experience has been that different people must take different paths
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to arrive at an understanding of RCU. This document provides several
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different paths, as follows:
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1. RCU OVERVIEW
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2. WHAT IS RCU'S CORE API?
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3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
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4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
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5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
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6. ANALOGY WITH READER-WRITER LOCKING
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7. FULL LIST OF RCU APIs
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8. ANSWERS TO QUICK QUIZZES
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People who prefer starting with a conceptual overview should focus on
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Section 1, though most readers will profit by reading this section at
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some point. People who prefer to start with an API that they can then
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experiment with should focus on Section 2. People who prefer to start
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with example uses should focus on Sections 3 and 4. People who need to
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understand the RCU implementation should focus on Section 5, then dive
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into the kernel source code. People who reason best by analogy should
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focus on Section 6. Section 7 serves as an index to the docbook API
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documentation, and Section 8 is the traditional answer key.
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So, start with the section that makes the most sense to you and your
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preferred method of learning. If you need to know everything about
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everything, feel free to read the whole thing -- but if you are really
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that type of person, you have perused the source code and will therefore
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never need this document anyway. ;-)
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1. RCU OVERVIEW
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The basic idea behind RCU is to split updates into "removal" and
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"reclamation" phases. The removal phase removes references to data items
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within a data structure (possibly by replacing them with references to
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new versions of these data items), and can run concurrently with readers.
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The reason that it is safe to run the removal phase concurrently with
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readers is the semantics of modern CPUs guarantee that readers will see
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either the old or the new version of the data structure rather than a
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partially updated reference. The reclamation phase does the work of reclaiming
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(e.g., freeing) the data items removed from the data structure during the
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removal phase. Because reclaiming data items can disrupt any readers
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concurrently referencing those data items, the reclamation phase must
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not start until readers no longer hold references to those data items.
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Splitting the update into removal and reclamation phases permits the
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updater to perform the removal phase immediately, and to defer the
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reclamation phase until all readers active during the removal phase have
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completed, either by blocking until they finish or by registering a
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callback that is invoked after they finish. Only readers that are active
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during the removal phase need be considered, because any reader starting
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after the removal phase will be unable to gain a reference to the removed
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data items, and therefore cannot be disrupted by the reclamation phase.
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So the typical RCU update sequence goes something like the following:
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a. Remove pointers to a data structure, so that subsequent
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readers cannot gain a reference to it.
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b. Wait for all previous readers to complete their RCU read-side
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critical sections.
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c. At this point, there cannot be any readers who hold references
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to the data structure, so it now may safely be reclaimed
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(e.g., kfree()d).
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Step (b) above is the key idea underlying RCU's deferred destruction.
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The ability to wait until all readers are done allows RCU readers to
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use much lighter-weight synchronization, in some cases, absolutely no
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synchronization at all. In contrast, in more conventional lock-based
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schemes, readers must use heavy-weight synchronization in order to
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prevent an updater from deleting the data structure out from under them.
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This is because lock-based updaters typically update data items in place,
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and must therefore exclude readers. In contrast, RCU-based updaters
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typically take advantage of the fact that writes to single aligned
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pointers are atomic on modern CPUs, allowing atomic insertion, removal,
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and replacement of data items in a linked structure without disrupting
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readers. Concurrent RCU readers can then continue accessing the old
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versions, and can dispense with the atomic operations, memory barriers,
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and communications cache misses that are so expensive on present-day
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SMP computer systems, even in absence of lock contention.
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In the three-step procedure shown above, the updater is performing both
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the removal and the reclamation step, but it is often helpful for an
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entirely different thread to do the reclamation, as is in fact the case
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in the Linux kernel's directory-entry cache (dcache). Even if the same
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thread performs both the update step (step (a) above) and the reclamation
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step (step (c) above), it is often helpful to think of them separately.
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For example, RCU readers and updaters need not communicate at all,
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but RCU provides implicit low-overhead communication between readers
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and reclaimers, namely, in step (b) above.
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So how the heck can a reclaimer tell when a reader is done, given
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that readers are not doing any sort of synchronization operations???
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Read on to learn about how RCU's API makes this easy.
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2. WHAT IS RCU'S CORE API?
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The core RCU API is quite small:
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a. rcu_read_lock()
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b. rcu_read_unlock()
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c. synchronize_rcu() / call_rcu()
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d. rcu_assign_pointer()
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e. rcu_dereference()
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There are many other members of the RCU API, but the rest can be
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expressed in terms of these five, though most implementations instead
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express synchronize_rcu() in terms of the call_rcu() callback API.
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The five core RCU APIs are described below, the other 18 will be enumerated
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later. See the kernel docbook documentation for more info, or look directly
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at the function header comments.
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rcu_read_lock()
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void rcu_read_lock(void);
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Used by a reader to inform the reclaimer that the reader is
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entering an RCU read-side critical section. It is illegal
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to block while in an RCU read-side critical section, though
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kernels built with CONFIG_TREE_PREEMPT_RCU can preempt RCU
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read-side critical sections. Any RCU-protected data structure
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accessed during an RCU read-side critical section is guaranteed to
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remain unreclaimed for the full duration of that critical section.
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Reference counts may be used in conjunction with RCU to maintain
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longer-term references to data structures.
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rcu_read_unlock()
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void rcu_read_unlock(void);
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Used by a reader to inform the reclaimer that the reader is
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exiting an RCU read-side critical section. Note that RCU
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read-side critical sections may be nested and/or overlapping.
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synchronize_rcu()
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void synchronize_rcu(void);
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Marks the end of updater code and the beginning of reclaimer
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code. It does this by blocking until all pre-existing RCU
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read-side critical sections on all CPUs have completed.
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Note that synchronize_rcu() will -not- necessarily wait for
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any subsequent RCU read-side critical sections to complete.
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For example, consider the following sequence of events:
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CPU 0 CPU 1 CPU 2
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----------------- ------------------------- ---------------
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1. rcu_read_lock()
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2. enters synchronize_rcu()
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3. rcu_read_lock()
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4. rcu_read_unlock()
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5. exits synchronize_rcu()
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6. rcu_read_unlock()
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To reiterate, synchronize_rcu() waits only for ongoing RCU
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read-side critical sections to complete, not necessarily for
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any that begin after synchronize_rcu() is invoked.
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Of course, synchronize_rcu() does not necessarily return
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-immediately- after the last pre-existing RCU read-side critical
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section completes. For one thing, there might well be scheduling
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delays. For another thing, many RCU implementations process
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requests in batches in order to improve efficiencies, which can
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further delay synchronize_rcu().
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Since synchronize_rcu() is the API that must figure out when
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readers are done, its implementation is key to RCU. For RCU
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to be useful in all but the most read-intensive situations,
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synchronize_rcu()'s overhead must also be quite small.
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The call_rcu() API is a callback form of synchronize_rcu(),
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and is described in more detail in a later section. Instead of
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blocking, it registers a function and argument which are invoked
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after all ongoing RCU read-side critical sections have completed.
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This callback variant is particularly useful in situations where
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it is illegal to block or where update-side performance is
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critically important.
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However, the call_rcu() API should not be used lightly, as use
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of the synchronize_rcu() API generally results in simpler code.
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In addition, the synchronize_rcu() API has the nice property
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of automatically limiting update rate should grace periods
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be delayed. This property results in system resilience in face
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of denial-of-service attacks. Code using call_rcu() should limit
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update rate in order to gain this same sort of resilience. See
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checklist.txt for some approaches to limiting the update rate.
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rcu_assign_pointer()
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typeof(p) rcu_assign_pointer(p, typeof(p) v);
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Yes, rcu_assign_pointer() -is- implemented as a macro, though it
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would be cool to be able to declare a function in this manner.
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(Compiler experts will no doubt disagree.)
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The updater uses this function to assign a new value to an
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RCU-protected pointer, in order to safely communicate the change
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in value from the updater to the reader. This function returns
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the new value, and also executes any memory-barrier instructions
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required for a given CPU architecture.
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Perhaps just as important, it serves to document (1) which
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pointers are protected by RCU and (2) the point at which a
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given structure becomes accessible to other CPUs. That said,
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rcu_assign_pointer() is most frequently used indirectly, via
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the _rcu list-manipulation primitives such as list_add_rcu().
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rcu_dereference()
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typeof(p) rcu_dereference(p);
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Like rcu_assign_pointer(), rcu_dereference() must be implemented
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as a macro.
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The reader uses rcu_dereference() to fetch an RCU-protected
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pointer, which returns a value that may then be safely
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dereferenced. Note that rcu_deference() does not actually
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dereference the pointer, instead, it protects the pointer for
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later dereferencing. It also executes any needed memory-barrier
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instructions for a given CPU architecture. Currently, only Alpha
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needs memory barriers within rcu_dereference() -- on other CPUs,
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it compiles to nothing, not even a compiler directive.
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Common coding practice uses rcu_dereference() to copy an
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RCU-protected pointer to a local variable, then dereferences
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this local variable, for example as follows:
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p = rcu_dereference(head.next);
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return p->data;
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However, in this case, one could just as easily combine these
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into one statement:
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return rcu_dereference(head.next)->data;
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If you are going to be fetching multiple fields from the
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RCU-protected structure, using the local variable is of
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course preferred. Repeated rcu_dereference() calls look
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ugly and incur unnecessary overhead on Alpha CPUs.
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Note that the value returned by rcu_dereference() is valid
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only within the enclosing RCU read-side critical section.
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For example, the following is -not- legal:
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rcu_read_lock();
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p = rcu_dereference(head.next);
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rcu_read_unlock();
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x = p->address; /* BUG!!! */
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rcu_read_lock();
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y = p->data; /* BUG!!! */
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rcu_read_unlock();
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Holding a reference from one RCU read-side critical section
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to another is just as illegal as holding a reference from
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one lock-based critical section to another! Similarly,
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using a reference outside of the critical section in which
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it was acquired is just as illegal as doing so with normal
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locking.
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As with rcu_assign_pointer(), an important function of
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rcu_dereference() is to document which pointers are protected by
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RCU, in particular, flagging a pointer that is subject to changing
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at any time, including immediately after the rcu_dereference().
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And, again like rcu_assign_pointer(), rcu_dereference() is
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typically used indirectly, via the _rcu list-manipulation
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primitives, such as list_for_each_entry_rcu().
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The following diagram shows how each API communicates among the
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reader, updater, and reclaimer.
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rcu_assign_pointer()
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+--------+
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+---------------------->| reader |---------+
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| +--------+ |
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| | |
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| | | Protect:
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| | | rcu_read_lock()
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| | | rcu_read_unlock()
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| rcu_dereference() | |
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+---------+ | |
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| updater |<---------------------+ |
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+---------+ V
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| +-----------+
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+----------------------------------->| reclaimer |
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+-----------+
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Defer:
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synchronize_rcu() & call_rcu()
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The RCU infrastructure observes the time sequence of rcu_read_lock(),
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rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
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order to determine when (1) synchronize_rcu() invocations may return
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to their callers and (2) call_rcu() callbacks may be invoked. Efficient
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implementations of the RCU infrastructure make heavy use of batching in
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order to amortize their overhead over many uses of the corresponding APIs.
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There are no fewer than three RCU mechanisms in the Linux kernel; the
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diagram above shows the first one, which is by far the most commonly used.
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The rcu_dereference() and rcu_assign_pointer() primitives are used for
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all three mechanisms, but different defer and protect primitives are
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used as follows:
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Defer Protect
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a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
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call_rcu() rcu_dereference()
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b. synchronize_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
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call_rcu_bh() rcu_dereference_bh()
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c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
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call_rcu_sched() preempt_disable() / preempt_enable()
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local_irq_save() / local_irq_restore()
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hardirq enter / hardirq exit
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NMI enter / NMI exit
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rcu_dereference_sched()
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These three mechanisms are used as follows:
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a. RCU applied to normal data structures.
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b. RCU applied to networking data structures that may be subjected
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to remote denial-of-service attacks.
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c. RCU applied to scheduler and interrupt/NMI-handler tasks.
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Again, most uses will be of (a). The (b) and (c) cases are important
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for specialized uses, but are relatively uncommon.
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3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
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This section shows a simple use of the core RCU API to protect a
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global pointer to a dynamically allocated structure. More-typical
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uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
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struct foo {
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int a;
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char b;
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long c;
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};
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DEFINE_SPINLOCK(foo_mutex);
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struct foo *gbl_foo;
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/*
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* Create a new struct foo that is the same as the one currently
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* pointed to by gbl_foo, except that field "a" is replaced
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* with "new_a". Points gbl_foo to the new structure, and
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* frees up the old structure after a grace period.
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*
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* Uses rcu_assign_pointer() to ensure that concurrent readers
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* see the initialized version of the new structure.
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*
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* Uses synchronize_rcu() to ensure that any readers that might
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* have references to the old structure complete before freeing
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* the old structure.
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*/
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void foo_update_a(int new_a)
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{
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struct foo *new_fp;
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struct foo *old_fp;
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new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
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spin_lock(&foo_mutex);
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old_fp = gbl_foo;
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*new_fp = *old_fp;
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new_fp->a = new_a;
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rcu_assign_pointer(gbl_foo, new_fp);
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spin_unlock(&foo_mutex);
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synchronize_rcu();
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kfree(old_fp);
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}
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/*
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* Return the value of field "a" of the current gbl_foo
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* structure. Use rcu_read_lock() and rcu_read_unlock()
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* to ensure that the structure does not get deleted out
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* from under us, and use rcu_dereference() to ensure that
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* we see the initialized version of the structure (important
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* for DEC Alpha and for people reading the code).
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*/
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int foo_get_a(void)
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{
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int retval;
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rcu_read_lock();
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retval = rcu_dereference(gbl_foo)->a;
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rcu_read_unlock();
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return retval;
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}
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So, to sum up:
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o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
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read-side critical sections.
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o Within an RCU read-side critical section, use rcu_dereference()
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to dereference RCU-protected pointers.
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o Use some solid scheme (such as locks or semaphores) to
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keep concurrent updates from interfering with each other.
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o Use rcu_assign_pointer() to update an RCU-protected pointer.
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This primitive protects concurrent readers from the updater,
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-not- concurrent updates from each other! You therefore still
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need to use locking (or something similar) to keep concurrent
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rcu_assign_pointer() primitives from interfering with each other.
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o Use synchronize_rcu() -after- removing a data element from an
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RCU-protected data structure, but -before- reclaiming/freeing
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the data element, in order to wait for the completion of all
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RCU read-side critical sections that might be referencing that
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data item.
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See checklist.txt for additional rules to follow when using RCU.
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And again, more-typical uses of RCU may be found in listRCU.txt,
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arrayRCU.txt, and NMI-RCU.txt.
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4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
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In the example above, foo_update_a() blocks until a grace period elapses.
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This is quite simple, but in some cases one cannot afford to wait so
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long -- there might be other high-priority work to be done.
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In such cases, one uses call_rcu() rather than synchronize_rcu().
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The call_rcu() API is as follows:
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void call_rcu(struct rcu_head * head,
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void (*func)(struct rcu_head *head));
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This function invokes func(head) after a grace period has elapsed.
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This invocation might happen from either softirq or process context,
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so the function is not permitted to block. The foo struct needs to
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have an rcu_head structure added, perhaps as follows:
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struct foo {
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int a;
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char b;
|
|
long c;
|
|
struct rcu_head rcu;
|
|
};
|
|
|
|
The foo_update_a() function might then be written as follows:
|
|
|
|
/*
|
|
* Create a new struct foo that is the same as the one currently
|
|
* pointed to by gbl_foo, except that field "a" is replaced
|
|
* with "new_a". Points gbl_foo to the new structure, and
|
|
* frees up the old structure after a grace period.
|
|
*
|
|
* Uses rcu_assign_pointer() to ensure that concurrent readers
|
|
* see the initialized version of the new structure.
|
|
*
|
|
* Uses call_rcu() to ensure that any readers that might have
|
|
* references to the old structure complete before freeing the
|
|
* old structure.
|
|
*/
|
|
void foo_update_a(int new_a)
|
|
{
|
|
struct foo *new_fp;
|
|
struct foo *old_fp;
|
|
|
|
new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
|
|
spin_lock(&foo_mutex);
|
|
old_fp = gbl_foo;
|
|
*new_fp = *old_fp;
|
|
new_fp->a = new_a;
|
|
rcu_assign_pointer(gbl_foo, new_fp);
|
|
spin_unlock(&foo_mutex);
|
|
call_rcu(&old_fp->rcu, foo_reclaim);
|
|
}
|
|
|
|
The foo_reclaim() function might appear as follows:
|
|
|
|
void foo_reclaim(struct rcu_head *rp)
|
|
{
|
|
struct foo *fp = container_of(rp, struct foo, rcu);
|
|
|
|
foo_cleanup(fp->a);
|
|
|
|
kfree(fp);
|
|
}
|
|
|
|
The container_of() primitive is a macro that, given a pointer into a
|
|
struct, the type of the struct, and the pointed-to field within the
|
|
struct, returns a pointer to the beginning of the struct.
|
|
|
|
The use of call_rcu() permits the caller of foo_update_a() to
|
|
immediately regain control, without needing to worry further about the
|
|
old version of the newly updated element. It also clearly shows the
|
|
RCU distinction between updater, namely foo_update_a(), and reclaimer,
|
|
namely foo_reclaim().
|
|
|
|
The summary of advice is the same as for the previous section, except
|
|
that we are now using call_rcu() rather than synchronize_rcu():
|
|
|
|
o Use call_rcu() -after- removing a data element from an
|
|
RCU-protected data structure in order to register a callback
|
|
function that will be invoked after the completion of all RCU
|
|
read-side critical sections that might be referencing that
|
|
data item.
|
|
|
|
If the callback for call_rcu() is not doing anything more than calling
|
|
kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
|
|
to avoid having to write your own callback:
|
|
|
|
kfree_rcu(old_fp, rcu);
|
|
|
|
Again, see checklist.txt for additional rules governing the use of RCU.
|
|
|
|
|
|
5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
|
|
|
|
One of the nice things about RCU is that it has extremely simple "toy"
|
|
implementations that are a good first step towards understanding the
|
|
production-quality implementations in the Linux kernel. This section
|
|
presents two such "toy" implementations of RCU, one that is implemented
|
|
in terms of familiar locking primitives, and another that more closely
|
|
resembles "classic" RCU. Both are way too simple for real-world use,
|
|
lacking both functionality and performance. However, they are useful
|
|
in getting a feel for how RCU works. See kernel/rcupdate.c for a
|
|
production-quality implementation, and see:
|
|
|
|
http://www.rdrop.com/users/paulmck/RCU
|
|
|
|
for papers describing the Linux kernel RCU implementation. The OLS'01
|
|
and OLS'02 papers are a good introduction, and the dissertation provides
|
|
more details on the current implementation as of early 2004.
|
|
|
|
|
|
5A. "TOY" IMPLEMENTATION #1: LOCKING
|
|
|
|
This section presents a "toy" RCU implementation that is based on
|
|
familiar locking primitives. Its overhead makes it a non-starter for
|
|
real-life use, as does its lack of scalability. It is also unsuitable
|
|
for realtime use, since it allows scheduling latency to "bleed" from
|
|
one read-side critical section to another.
|
|
|
|
However, it is probably the easiest implementation to relate to, so is
|
|
a good starting point.
|
|
|
|
It is extremely simple:
|
|
|
|
static DEFINE_RWLOCK(rcu_gp_mutex);
|
|
|
|
void rcu_read_lock(void)
|
|
{
|
|
read_lock(&rcu_gp_mutex);
|
|
}
|
|
|
|
void rcu_read_unlock(void)
|
|
{
|
|
read_unlock(&rcu_gp_mutex);
|
|
}
|
|
|
|
void synchronize_rcu(void)
|
|
{
|
|
write_lock(&rcu_gp_mutex);
|
|
write_unlock(&rcu_gp_mutex);
|
|
}
|
|
|
|
[You can ignore rcu_assign_pointer() and rcu_dereference() without
|
|
missing much. But here they are anyway. And whatever you do, don't
|
|
forget about them when submitting patches making use of RCU!]
|
|
|
|
#define rcu_assign_pointer(p, v) ({ \
|
|
smp_wmb(); \
|
|
(p) = (v); \
|
|
})
|
|
|
|
#define rcu_dereference(p) ({ \
|
|
typeof(p) _________p1 = p; \
|
|
smp_read_barrier_depends(); \
|
|
(_________p1); \
|
|
})
|
|
|
|
|
|
The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
|
|
and release a global reader-writer lock. The synchronize_rcu()
|
|
primitive write-acquires this same lock, then immediately releases
|
|
it. This means that once synchronize_rcu() exits, all RCU read-side
|
|
critical sections that were in progress before synchronize_rcu() was
|
|
called are guaranteed to have completed -- there is no way that
|
|
synchronize_rcu() would have been able to write-acquire the lock
|
|
otherwise.
|
|
|
|
It is possible to nest rcu_read_lock(), since reader-writer locks may
|
|
be recursively acquired. Note also that rcu_read_lock() is immune
|
|
from deadlock (an important property of RCU). The reason for this is
|
|
that the only thing that can block rcu_read_lock() is a synchronize_rcu().
|
|
But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
|
|
so there can be no deadlock cycle.
|
|
|
|
Quick Quiz #1: Why is this argument naive? How could a deadlock
|
|
occur when using this algorithm in a real-world Linux
|
|
kernel? How could this deadlock be avoided?
|
|
|
|
|
|
5B. "TOY" EXAMPLE #2: CLASSIC RCU
|
|
|
|
This section presents a "toy" RCU implementation that is based on
|
|
"classic RCU". It is also short on performance (but only for updates) and
|
|
on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
|
|
kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
|
|
are the same as those shown in the preceding section, so they are omitted.
|
|
|
|
void rcu_read_lock(void) { }
|
|
|
|
void rcu_read_unlock(void) { }
|
|
|
|
void synchronize_rcu(void)
|
|
{
|
|
int cpu;
|
|
|
|
for_each_possible_cpu(cpu)
|
|
run_on(cpu);
|
|
}
|
|
|
|
Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
|
|
This is the great strength of classic RCU in a non-preemptive kernel:
|
|
read-side overhead is precisely zero, at least on non-Alpha CPUs.
|
|
And there is absolutely no way that rcu_read_lock() can possibly
|
|
participate in a deadlock cycle!
|
|
|
|
The implementation of synchronize_rcu() simply schedules itself on each
|
|
CPU in turn. The run_on() primitive can be implemented straightforwardly
|
|
in terms of the sched_setaffinity() primitive. Of course, a somewhat less
|
|
"toy" implementation would restore the affinity upon completion rather
|
|
than just leaving all tasks running on the last CPU, but when I said
|
|
"toy", I meant -toy-!
|
|
|
|
So how the heck is this supposed to work???
|
|
|
|
Remember that it is illegal to block while in an RCU read-side critical
|
|
section. Therefore, if a given CPU executes a context switch, we know
|
|
that it must have completed all preceding RCU read-side critical sections.
|
|
Once -all- CPUs have executed a context switch, then -all- preceding
|
|
RCU read-side critical sections will have completed.
|
|
|
|
So, suppose that we remove a data item from its structure and then invoke
|
|
synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
|
|
that there are no RCU read-side critical sections holding a reference
|
|
to that data item, so we can safely reclaim it.
|
|
|
|
Quick Quiz #2: Give an example where Classic RCU's read-side
|
|
overhead is -negative-.
|
|
|
|
Quick Quiz #3: If it is illegal to block in an RCU read-side
|
|
critical section, what the heck do you do in
|
|
PREEMPT_RT, where normal spinlocks can block???
|
|
|
|
|
|
6. ANALOGY WITH READER-WRITER LOCKING
|
|
|
|
Although RCU can be used in many different ways, a very common use of
|
|
RCU is analogous to reader-writer locking. The following unified
|
|
diff shows how closely related RCU and reader-writer locking can be.
|
|
|
|
@@ -13,15 +14,15 @@
|
|
struct list_head *lp;
|
|
struct el *p;
|
|
|
|
- read_lock();
|
|
- list_for_each_entry(p, head, lp) {
|
|
+ rcu_read_lock();
|
|
+ list_for_each_entry_rcu(p, head, lp) {
|
|
if (p->key == key) {
|
|
*result = p->data;
|
|
- read_unlock();
|
|
+ rcu_read_unlock();
|
|
return 1;
|
|
}
|
|
}
|
|
- read_unlock();
|
|
+ rcu_read_unlock();
|
|
return 0;
|
|
}
|
|
|
|
@@ -29,15 +30,16 @@
|
|
{
|
|
struct el *p;
|
|
|
|
- write_lock(&listmutex);
|
|
+ spin_lock(&listmutex);
|
|
list_for_each_entry(p, head, lp) {
|
|
if (p->key == key) {
|
|
- list_del(&p->list);
|
|
- write_unlock(&listmutex);
|
|
+ list_del_rcu(&p->list);
|
|
+ spin_unlock(&listmutex);
|
|
+ synchronize_rcu();
|
|
kfree(p);
|
|
return 1;
|
|
}
|
|
}
|
|
- write_unlock(&listmutex);
|
|
+ spin_unlock(&listmutex);
|
|
return 0;
|
|
}
|
|
|
|
Or, for those who prefer a side-by-side listing:
|
|
|
|
1 struct el { 1 struct el {
|
|
2 struct list_head list; 2 struct list_head list;
|
|
3 long key; 3 long key;
|
|
4 spinlock_t mutex; 4 spinlock_t mutex;
|
|
5 int data; 5 int data;
|
|
6 /* Other data fields */ 6 /* Other data fields */
|
|
7 }; 7 };
|
|
8 spinlock_t listmutex; 8 spinlock_t listmutex;
|
|
9 struct el head; 9 struct el head;
|
|
|
|
1 int search(long key, int *result) 1 int search(long key, int *result)
|
|
2 { 2 {
|
|
3 struct list_head *lp; 3 struct list_head *lp;
|
|
4 struct el *p; 4 struct el *p;
|
|
5 5
|
|
6 read_lock(); 6 rcu_read_lock();
|
|
7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
|
|
8 if (p->key == key) { 8 if (p->key == key) {
|
|
9 *result = p->data; 9 *result = p->data;
|
|
10 read_unlock(); 10 rcu_read_unlock();
|
|
11 return 1; 11 return 1;
|
|
12 } 12 }
|
|
13 } 13 }
|
|
14 read_unlock(); 14 rcu_read_unlock();
|
|
15 return 0; 15 return 0;
|
|
16 } 16 }
|
|
|
|
1 int delete(long key) 1 int delete(long key)
|
|
2 { 2 {
|
|
3 struct el *p; 3 struct el *p;
|
|
4 4
|
|
5 write_lock(&listmutex); 5 spin_lock(&listmutex);
|
|
6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
|
|
7 if (p->key == key) { 7 if (p->key == key) {
|
|
8 list_del(&p->list); 8 list_del_rcu(&p->list);
|
|
9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
|
|
10 synchronize_rcu();
|
|
10 kfree(p); 11 kfree(p);
|
|
11 return 1; 12 return 1;
|
|
12 } 13 }
|
|
13 } 14 }
|
|
14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
|
|
15 return 0; 16 return 0;
|
|
16 } 17 }
|
|
|
|
Either way, the differences are quite small. Read-side locking moves
|
|
to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
|
|
a reader-writer lock to a simple spinlock, and a synchronize_rcu()
|
|
precedes the kfree().
|
|
|
|
However, there is one potential catch: the read-side and update-side
|
|
critical sections can now run concurrently. In many cases, this will
|
|
not be a problem, but it is necessary to check carefully regardless.
|
|
For example, if multiple independent list updates must be seen as
|
|
a single atomic update, converting to RCU will require special care.
|
|
|
|
Also, the presence of synchronize_rcu() means that the RCU version of
|
|
delete() can now block. If this is a problem, there is a callback-based
|
|
mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
|
|
be used in place of synchronize_rcu().
|
|
|
|
|
|
7. FULL LIST OF RCU APIs
|
|
|
|
The RCU APIs are documented in docbook-format header comments in the
|
|
Linux-kernel source code, but it helps to have a full list of the
|
|
APIs, since there does not appear to be a way to categorize them
|
|
in docbook. Here is the list, by category.
|
|
|
|
RCU list traversal:
|
|
|
|
list_entry_rcu
|
|
list_first_entry_rcu
|
|
list_next_rcu
|
|
list_for_each_entry_rcu
|
|
list_for_each_entry_continue_rcu
|
|
hlist_first_rcu
|
|
hlist_next_rcu
|
|
hlist_pprev_rcu
|
|
hlist_for_each_entry_rcu
|
|
hlist_for_each_entry_rcu_bh
|
|
hlist_for_each_entry_continue_rcu
|
|
hlist_for_each_entry_continue_rcu_bh
|
|
hlist_nulls_first_rcu
|
|
hlist_nulls_for_each_entry_rcu
|
|
hlist_bl_first_rcu
|
|
hlist_bl_for_each_entry_rcu
|
|
|
|
RCU pointer/list update:
|
|
|
|
rcu_assign_pointer
|
|
list_add_rcu
|
|
list_add_tail_rcu
|
|
list_del_rcu
|
|
list_replace_rcu
|
|
hlist_add_behind_rcu
|
|
hlist_add_before_rcu
|
|
hlist_add_head_rcu
|
|
hlist_del_rcu
|
|
hlist_del_init_rcu
|
|
hlist_replace_rcu
|
|
list_splice_init_rcu()
|
|
hlist_nulls_del_init_rcu
|
|
hlist_nulls_del_rcu
|
|
hlist_nulls_add_head_rcu
|
|
hlist_bl_add_head_rcu
|
|
hlist_bl_del_init_rcu
|
|
hlist_bl_del_rcu
|
|
hlist_bl_set_first_rcu
|
|
|
|
RCU: Critical sections Grace period Barrier
|
|
|
|
rcu_read_lock synchronize_net rcu_barrier
|
|
rcu_read_unlock synchronize_rcu
|
|
rcu_dereference synchronize_rcu_expedited
|
|
rcu_read_lock_held call_rcu
|
|
rcu_dereference_check kfree_rcu
|
|
rcu_dereference_protected
|
|
|
|
bh: Critical sections Grace period Barrier
|
|
|
|
rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
|
|
rcu_read_unlock_bh synchronize_rcu_bh
|
|
rcu_dereference_bh synchronize_rcu_bh_expedited
|
|
rcu_dereference_bh_check
|
|
rcu_dereference_bh_protected
|
|
rcu_read_lock_bh_held
|
|
|
|
sched: Critical sections Grace period Barrier
|
|
|
|
rcu_read_lock_sched synchronize_sched rcu_barrier_sched
|
|
rcu_read_unlock_sched call_rcu_sched
|
|
[preempt_disable] synchronize_sched_expedited
|
|
[and friends]
|
|
rcu_read_lock_sched_notrace
|
|
rcu_read_unlock_sched_notrace
|
|
rcu_dereference_sched
|
|
rcu_dereference_sched_check
|
|
rcu_dereference_sched_protected
|
|
rcu_read_lock_sched_held
|
|
|
|
|
|
SRCU: Critical sections Grace period Barrier
|
|
|
|
srcu_read_lock synchronize_srcu srcu_barrier
|
|
srcu_read_unlock call_srcu
|
|
srcu_dereference synchronize_srcu_expedited
|
|
srcu_dereference_check
|
|
srcu_read_lock_held
|
|
|
|
SRCU: Initialization/cleanup
|
|
init_srcu_struct
|
|
cleanup_srcu_struct
|
|
|
|
All: lockdep-checked RCU-protected pointer access
|
|
|
|
rcu_access_index
|
|
rcu_access_pointer
|
|
rcu_dereference_index_check
|
|
rcu_dereference_raw
|
|
rcu_lockdep_assert
|
|
rcu_sleep_check
|
|
RCU_NONIDLE
|
|
|
|
See the comment headers in the source code (or the docbook generated
|
|
from them) for more information.
|
|
|
|
However, given that there are no fewer than four families of RCU APIs
|
|
in the Linux kernel, how do you choose which one to use? The following
|
|
list can be helpful:
|
|
|
|
a. Will readers need to block? If so, you need SRCU.
|
|
|
|
b. What about the -rt patchset? If readers would need to block
|
|
in an non-rt kernel, you need SRCU. If readers would block
|
|
in a -rt kernel, but not in a non-rt kernel, SRCU is not
|
|
necessary.
|
|
|
|
c. Do you need to treat NMI handlers, hardirq handlers,
|
|
and code segments with preemption disabled (whether
|
|
via preempt_disable(), local_irq_save(), local_bh_disable(),
|
|
or some other mechanism) as if they were explicit RCU readers?
|
|
If so, RCU-sched is the only choice that will work for you.
|
|
|
|
d. Do you need RCU grace periods to complete even in the face
|
|
of softirq monopolization of one or more of the CPUs? For
|
|
example, is your code subject to network-based denial-of-service
|
|
attacks? If so, you need RCU-bh.
|
|
|
|
e. Is your workload too update-intensive for normal use of
|
|
RCU, but inappropriate for other synchronization mechanisms?
|
|
If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
|
|
|
|
f. Do you need read-side critical sections that are respected
|
|
even though they are in the middle of the idle loop, during
|
|
user-mode execution, or on an offlined CPU? If so, SRCU is the
|
|
only choice that will work for you.
|
|
|
|
g. Otherwise, use RCU.
|
|
|
|
Of course, this all assumes that you have determined that RCU is in fact
|
|
the right tool for your job.
|
|
|
|
|
|
8. ANSWERS TO QUICK QUIZZES
|
|
|
|
Quick Quiz #1: Why is this argument naive? How could a deadlock
|
|
occur when using this algorithm in a real-world Linux
|
|
kernel? [Referring to the lock-based "toy" RCU
|
|
algorithm.]
|
|
|
|
Answer: Consider the following sequence of events:
|
|
|
|
1. CPU 0 acquires some unrelated lock, call it
|
|
"problematic_lock", disabling irq via
|
|
spin_lock_irqsave().
|
|
|
|
2. CPU 1 enters synchronize_rcu(), write-acquiring
|
|
rcu_gp_mutex.
|
|
|
|
3. CPU 0 enters rcu_read_lock(), but must wait
|
|
because CPU 1 holds rcu_gp_mutex.
|
|
|
|
4. CPU 1 is interrupted, and the irq handler
|
|
attempts to acquire problematic_lock.
|
|
|
|
The system is now deadlocked.
|
|
|
|
One way to avoid this deadlock is to use an approach like
|
|
that of CONFIG_PREEMPT_RT, where all normal spinlocks
|
|
become blocking locks, and all irq handlers execute in
|
|
the context of special tasks. In this case, in step 4
|
|
above, the irq handler would block, allowing CPU 1 to
|
|
release rcu_gp_mutex, avoiding the deadlock.
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Even in the absence of deadlock, this RCU implementation
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allows latency to "bleed" from readers to other
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readers through synchronize_rcu(). To see this,
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consider task A in an RCU read-side critical section
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(thus read-holding rcu_gp_mutex), task B blocked
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attempting to write-acquire rcu_gp_mutex, and
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task C blocked in rcu_read_lock() attempting to
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read_acquire rcu_gp_mutex. Task A's RCU read-side
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latency is holding up task C, albeit indirectly via
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task B.
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Realtime RCU implementations therefore use a counter-based
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approach where tasks in RCU read-side critical sections
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cannot be blocked by tasks executing synchronize_rcu().
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Quick Quiz #2: Give an example where Classic RCU's read-side
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overhead is -negative-.
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Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
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kernel where a routing table is used by process-context
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code, but can be updated by irq-context code (for example,
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by an "ICMP REDIRECT" packet). The usual way of handling
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this would be to have the process-context code disable
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interrupts while searching the routing table. Use of
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RCU allows such interrupt-disabling to be dispensed with.
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Thus, without RCU, you pay the cost of disabling interrupts,
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and with RCU you don't.
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One can argue that the overhead of RCU in this
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case is negative with respect to the single-CPU
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interrupt-disabling approach. Others might argue that
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the overhead of RCU is merely zero, and that replacing
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the positive overhead of the interrupt-disabling scheme
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with the zero-overhead RCU scheme does not constitute
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negative overhead.
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In real life, of course, things are more complex. But
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even the theoretical possibility of negative overhead for
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a synchronization primitive is a bit unexpected. ;-)
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Quick Quiz #3: If it is illegal to block in an RCU read-side
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critical section, what the heck do you do in
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PREEMPT_RT, where normal spinlocks can block???
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Answer: Just as PREEMPT_RT permits preemption of spinlock
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critical sections, it permits preemption of RCU
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read-side critical sections. It also permits
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spinlocks blocking while in RCU read-side critical
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sections.
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Why the apparent inconsistency? Because it is it
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possible to use priority boosting to keep the RCU
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grace periods short if need be (for example, if running
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short of memory). In contrast, if blocking waiting
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for (say) network reception, there is no way to know
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what should be boosted. Especially given that the
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process we need to boost might well be a human being
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who just went out for a pizza or something. And although
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a computer-operated cattle prod might arouse serious
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interest, it might also provoke serious objections.
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Besides, how does the computer know what pizza parlor
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the human being went to???
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ACKNOWLEDGEMENTS
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My thanks to the people who helped make this human-readable, including
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Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
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For more information, see http://www.rdrop.com/users/paulmck/RCU.
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