mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-22 17:01:14 +00:00
![Randy Dunlap](/assets/img/avatar_default.png)
Change the phrase "at at least" to "to at least" to be more readable. Signed-off-by: Randy Dunlap <rdunlap@infradead.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: linux-doc@vger.kernel.org Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20200703213649.30948-2-rdunlap@infradead.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
152 lines
6 KiB
ReStructuredText
152 lines
6 KiB
ReStructuredText
=======================
|
|
Generic Mutex Subsystem
|
|
=======================
|
|
|
|
started by Ingo Molnar <mingo@redhat.com>
|
|
|
|
updated by Davidlohr Bueso <davidlohr@hp.com>
|
|
|
|
What are mutexes?
|
|
-----------------
|
|
|
|
In the Linux kernel, mutexes refer to a particular locking primitive
|
|
that enforces serialization on shared memory systems, and not only to
|
|
the generic term referring to 'mutual exclusion' found in academia
|
|
or similar theoretical text books. Mutexes are sleeping locks which
|
|
behave similarly to binary semaphores, and were introduced in 2006[1]
|
|
as an alternative to these. This new data structure provided a number
|
|
of advantages, including simpler interfaces, and at that time smaller
|
|
code (see Disadvantages).
|
|
|
|
[1] https://lwn.net/Articles/164802/
|
|
|
|
Implementation
|
|
--------------
|
|
|
|
Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h
|
|
and implemented in kernel/locking/mutex.c. These locks use an atomic variable
|
|
(->owner) to keep track of the lock state during its lifetime. Field owner
|
|
actually contains `struct task_struct *` to the current lock owner and it is
|
|
therefore NULL if not currently owned. Since task_struct pointers are aligned
|
|
to at least L1_CACHE_BYTES, low bits (3) are used to store extra state (e.g.,
|
|
if waiter list is non-empty). In its most basic form it also includes a
|
|
wait-queue and a spinlock that serializes access to it. Furthermore,
|
|
CONFIG_MUTEX_SPIN_ON_OWNER=y systems use a spinner MCS lock (->osq), described
|
|
below in (ii).
|
|
|
|
When acquiring a mutex, there are three possible paths that can be
|
|
taken, depending on the state of the lock:
|
|
|
|
(i) fastpath: tries to atomically acquire the lock by cmpxchg()ing the owner with
|
|
the current task. This only works in the uncontended case (cmpxchg() checks
|
|
against 0UL, so all 3 state bits above have to be 0). If the lock is
|
|
contended it goes to the next possible path.
|
|
|
|
(ii) midpath: aka optimistic spinning, tries to spin for acquisition
|
|
while the lock owner is running and there are no other tasks ready
|
|
to run that have higher priority (need_resched). The rationale is
|
|
that if the lock owner is running, it is likely to release the lock
|
|
soon. The mutex spinners are queued up using MCS lock so that only
|
|
one spinner can compete for the mutex.
|
|
|
|
The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spinlock
|
|
with the desirable properties of being fair and with each cpu trying
|
|
to acquire the lock spinning on a local variable. It avoids expensive
|
|
cacheline bouncing that common test-and-set spinlock implementations
|
|
incur. An MCS-like lock is specially tailored for optimistic spinning
|
|
for sleeping lock implementation. An important feature of the customized
|
|
MCS lock is that it has the extra property that spinners are able to exit
|
|
the MCS spinlock queue when they need to reschedule. This further helps
|
|
avoid situations where MCS spinners that need to reschedule would continue
|
|
waiting to spin on mutex owner, only to go directly to slowpath upon
|
|
obtaining the MCS lock.
|
|
|
|
|
|
(iii) slowpath: last resort, if the lock is still unable to be acquired,
|
|
the task is added to the wait-queue and sleeps until woken up by the
|
|
unlock path. Under normal circumstances it blocks as TASK_UNINTERRUPTIBLE.
|
|
|
|
While formally kernel mutexes are sleepable locks, it is path (ii) that
|
|
makes them more practically a hybrid type. By simply not interrupting a
|
|
task and busy-waiting for a few cycles instead of immediately sleeping,
|
|
the performance of this lock has been seen to significantly improve a
|
|
number of workloads. Note that this technique is also used for rw-semaphores.
|
|
|
|
Semantics
|
|
---------
|
|
|
|
The mutex subsystem checks and enforces the following rules:
|
|
|
|
- Only one task can hold the mutex at a time.
|
|
- Only the owner can unlock the mutex.
|
|
- Multiple unlocks are not permitted.
|
|
- Recursive locking/unlocking is not permitted.
|
|
- A mutex must only be initialized via the API (see below).
|
|
- A task may not exit with a mutex held.
|
|
- Memory areas where held locks reside must not be freed.
|
|
- Held mutexes must not be reinitialized.
|
|
- Mutexes may not be used in hardware or software interrupt
|
|
contexts such as tasklets and timers.
|
|
|
|
These semantics are fully enforced when CONFIG DEBUG_MUTEXES is enabled.
|
|
In addition, the mutex debugging code also implements a number of other
|
|
features that make lock debugging easier and faster:
|
|
|
|
- Uses symbolic names of mutexes, whenever they are printed
|
|
in debug output.
|
|
- Point-of-acquire tracking, symbolic lookup of function names,
|
|
list of all locks held in the system, printout of them.
|
|
- Owner tracking.
|
|
- Detects self-recursing locks and prints out all relevant info.
|
|
- Detects multi-task circular deadlocks and prints out all affected
|
|
locks and tasks (and only those tasks).
|
|
|
|
|
|
Interfaces
|
|
----------
|
|
Statically define the mutex::
|
|
|
|
DEFINE_MUTEX(name);
|
|
|
|
Dynamically initialize the mutex::
|
|
|
|
mutex_init(mutex);
|
|
|
|
Acquire the mutex, uninterruptible::
|
|
|
|
void mutex_lock(struct mutex *lock);
|
|
void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
|
|
int mutex_trylock(struct mutex *lock);
|
|
|
|
Acquire the mutex, interruptible::
|
|
|
|
int mutex_lock_interruptible_nested(struct mutex *lock,
|
|
unsigned int subclass);
|
|
int mutex_lock_interruptible(struct mutex *lock);
|
|
|
|
Acquire the mutex, interruptible, if dec to 0::
|
|
|
|
int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
|
|
|
|
Unlock the mutex::
|
|
|
|
void mutex_unlock(struct mutex *lock);
|
|
|
|
Test if the mutex is taken::
|
|
|
|
int mutex_is_locked(struct mutex *lock);
|
|
|
|
Disadvantages
|
|
-------------
|
|
|
|
Unlike its original design and purpose, 'struct mutex' is among the largest
|
|
locks in the kernel. E.g: on x86-64 it is 32 bytes, where 'struct semaphore'
|
|
is 24 bytes and rw_semaphore is 40 bytes. Larger structure sizes mean more CPU
|
|
cache and memory footprint.
|
|
|
|
When to use mutexes
|
|
-------------------
|
|
|
|
Unless the strict semantics of mutexes are unsuitable and/or the critical
|
|
region prevents the lock from being shared, always prefer them to any other
|
|
locking primitive.
|