Subtle race conditions can result if a CPU stays in dyntick-idle mode
long enough for the ->gpnum and ->completed fields to wrap. For
example, consider the following sequence of events:
o CPU 1 encounters a quiescent state while waiting for grace period
5 to complete, but then enters dyntick-idle mode.
o While CPU 1 is in dyntick-idle mode, the grace-period counters
wrap around so that the grace period number is now 4.
o Just as CPU 1 exits dyntick-idle mode, grace period 4 completes
and grace period 5 begins.
o The quiescent state that CPU 1 passed through during the old
grace period 5 looks like it applies to the new grace period
5. Therefore, the new grace period 5 completes without CPU 1
having passed through a quiescent state.
This could clearly be a fatal surprise to any long-running RCU read-side
critical section that happened to be running on CPU 1 at the time. At one
time, this was not a problem, given that it takes significant time for
the grace-period counters to overflow even on 32-bit systems. However,
with the advent of NO_HZ_FULL and SMP embedded systems, arbitrarily long
idle periods are now becoming quite feasible. It is therefore time to
close this race.
This commit therefore avoids this race condition by having the
quiescent-state forcing code detect when a CPU is falling too far
behind, and setting a new rcu_data field ->gpwrap when this happens.
Whenever this new ->gpwrap field is set, the CPU's ->gpnum and ->completed
fields are known to be untrustworthy, and can be ignored, along with
any associated quiescent states.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The current RCU CPU stall warning code will print "Stall ended before
state dump start" any time that the stall-warning code is triggered on
a CPU that has already reported a quiescent state for the current grace
period and if all quiescent states have been reported for the current
grace period. However, a true stall can result in these symptoms, for
example, by preventing RCU's grace-period kthreads from ever running
This commit therefore checks for this condition, reporting the end of
the stall only if one of the grace-period counters has actually advanced.
Otherwise, it reports the last time that the grace-period kthread made
meaningful progress. (In normal situations, the grace-period kthread
should make meaningful progress at least every jiffies_till_next_fqs
jiffies.)
Reported-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Miroslav Benes <mbenes@suse.cz>
One way that an RCU CPU stall warning can happen is if the grace-period
kthread is not allowed to execute. One proxy for this kthread's
forward progress is the number of force-quiescent-state (fqs) scans.
This commit therefore adds the number of fqs scans to the RCU CPU stall
warning printouts when CONFIG_RCU_CPU_STALL_INFO=y.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The x86 architecture has multiple types of NMI-like interrupts: real
NMIs, machine checks, and, for some values of NMI-like, debugging
and breakpoint interrupts. These interrupts can nest inside each
other. Andy Lutomirski is adding RCU support to these interrupts,
so rcu_nmi_enter() and rcu_nmi_exit() must now correctly handle nesting.
This commit therefore introduces nesting, using a clever NMI-coordination
algorithm suggested by Andy. The trick is to atomically increment
->dynticks (if needed) before manipulating ->dynticks_nmi_nesting on entry
(and, accordingly, after on exit). In addition, ->dynticks_nmi_nesting
is incremented by one if ->dynticks was incremented and by two otherwise.
This means that when rcu_nmi_exit() sees ->dynticks_nmi_nesting equal
to one, it knows that ->dynticks must be atomically incremented.
This NMI-coordination algorithms has been validated by the following
Promela model:
------------------------------------------------------------------------
/*
* Promela model for Andy Lutomirski's suggested change to rcu_nmi_enter()
* that allows nesting.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, you can access it online at
* http://www.gnu.org/licenses/gpl-2.0.html.
*
* Copyright IBM Corporation, 2014
*
* Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
*/
byte dynticks_nmi_nesting = 0;
byte dynticks = 0;
/*
* Promela verision of rcu_nmi_enter().
*/
inline rcu_nmi_enter()
{
byte incby;
byte tmp;
incby = BUSY_INCBY;
assert(dynticks_nmi_nesting >= 0);
if
:: (dynticks & 1) == 0 ->
atomic {
dynticks = dynticks + 1;
}
assert((dynticks & 1) == 1);
incby = 1;
:: else ->
skip;
fi;
tmp = dynticks_nmi_nesting;
tmp = tmp + incby;
dynticks_nmi_nesting = tmp;
assert(dynticks_nmi_nesting >= 1);
}
/*
* Promela verision of rcu_nmi_exit().
*/
inline rcu_nmi_exit()
{
byte tmp;
assert(dynticks_nmi_nesting > 0);
assert((dynticks & 1) != 0);
if
:: dynticks_nmi_nesting != 1 ->
tmp = dynticks_nmi_nesting;
tmp = tmp - BUSY_INCBY;
dynticks_nmi_nesting = tmp;
:: else ->
dynticks_nmi_nesting = 0;
atomic {
dynticks = dynticks + 1;
}
assert((dynticks & 1) == 0);
fi;
}
/*
* Base-level NMI runs non-atomically. Crudely emulates process-level
* dynticks-idle entry/exit.
*/
proctype base_NMI()
{
byte busy;
busy = 0;
do
:: /* Emulate base-level dynticks and not. */
if
:: 1 -> atomic {
dynticks = dynticks + 1;
}
busy = 1;
:: 1 -> skip;
fi;
/* Verify that we only sometimes have base-level dynticks. */
if
:: busy == 0 -> skip;
:: busy == 1 -> skip;
fi;
/* Model RCU's NMI entry and exit actions. */
rcu_nmi_enter();
assert((dynticks & 1) == 1);
rcu_nmi_exit();
/* Emulated re-entering base-level dynticks and not. */
if
:: !busy -> skip;
:: busy ->
atomic {
dynticks = dynticks + 1;
}
busy = 0;
fi;
/* We had better now be in dyntick-idle mode. */
assert((dynticks & 1) == 0);
od;
}
/*
* Nested NMI runs atomically to emulate interrupting base_level().
*/
proctype nested_NMI()
{
do
:: /*
* Use an atomic section to model a nested NMI. This is
* guaranteed to interleave into base_NMI() between a pair
* of base_NMI() statements, just as a nested NMI would.
*/
atomic {
/* Verify that we only sometimes are in dynticks. */
if
:: (dynticks & 1) == 0 -> skip;
:: (dynticks & 1) == 1 -> skip;
fi;
/* Model RCU's NMI entry and exit actions. */
rcu_nmi_enter();
assert((dynticks & 1) == 1);
rcu_nmi_exit();
}
od;
}
init {
run base_NMI();
run nested_NMI();
}
------------------------------------------------------------------------
The following script can be used to run this model if placed in
rcu_nmi.spin:
------------------------------------------------------------------------
if ! spin -a rcu_nmi.spin
then
echo Spin errors!!!
exit 1
fi
if ! cc -DSAFETY -o pan pan.c
then
echo Compilation errors!!!
exit 1
fi
./pan -m100000
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Add early boot self tests for RCU under CONFIG_PROVE_RCU.
Currently the only test is adding a dummy callback which increments a counter
which we then later verify after calling rcu_barrier*().
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The "cpu" argument to rcu_cleanup_after_idle() is always the current
CPU, so drop it. This moves the smp_processor_id() from the caller to
rcu_cleanup_after_idle(), saving argument-passing overhead. Again,
the anticipated cross-CPU uses of these functions has been replaced
by NO_HZ_FULL.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
The "cpu" argument to rcu_prepare_for_idle() is always the current
CPU, so drop it. This in turn allows two of the uses of "cpu" in
this function to be replaced with a this_cpu_ptr() and the third by
smp_processor_id(), replacing that of the call to rcu_prepare_for_idle().
Again, the anticipated cross-CPU uses of these functions has been replaced
by NO_HZ_FULL.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
The "cpu" argument to rcu_needs_cpu() is always the current CPU, so drop
it. This in turn allows the "cpu" argument to rcu_cpu_has_callbacks()
to be removed, which allows the uses of "cpu" in both functions to be
replaced with a this_cpu_ptr(). Again, the anticipated cross-CPU uses
of these functions has been replaced by NO_HZ_FULL.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
The "cpu" argument to rcu_note_context_switch() is always the current
CPU, so drop it. This in turn allows the "cpu" argument to
rcu_preempt_note_context_switch() to be removed, which allows the sole
use of "cpu" in both functions to be replaced with a this_cpu_ptr().
Again, the anticipated cross-CPU uses of these functions has been
replaced by NO_HZ_FULL.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
Because rcu_preempt_check_callbacks()'s argument is guaranteed to
always be the current CPU, drop the argument and replace per_cpu()
with __this_cpu_read().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
Because rcu_pending()'s argument is guaranteed to always be the current
CPU, drop the argument and replace per_cpu_ptr() with this_cpu_ptr().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
The "cpu" argument was kept around on the off-chance that RCU might
offload scheduler-clock interrupts. However, this offload approach
has been replaced by NO_HZ_FULL, which offloads -all- RCU processing
from qualifying CPUs. It is therefore time to remove the "cpu" argument
to rcu_check_callbacks(), which this commit does.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
The rcu_data per-CPU variable has a number of fields that are atomically
manipulated, potentially by any CPU. This situation can result in false
sharing with per-CPU variables that have the misfortune of being allocated
adjacent to rcu_data in memory. This commit therefore changes the
DEFINE_PER_CPU() to DEFINE_PER_CPU_SHARED_ALIGNED() in order to avoid
this false sharing.
Reported-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
For some functions in kernel/rcu/tree* the rdtp parameter is always
this_cpu_ptr(rdtp). Remove the parameter if constant and calculate the
pointer in function.
This will have the advantage that it is obvious that the address are
all per cpu offsets and thus it will enable the use of this_cpu_ops in
the future.
Signed-off-by: Christoph Lameter <cl@linux.com>
[ paulmck: Forward-ported to rcu/dev, whitespace adjustment. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
If a no-CBs CPU were to post an RCU callback with interrupts disabled
after it entered the idle loop for the last time, there might be no
deferred wakeup for the corresponding rcuo kthreads. This commit
therefore adds a set of calls to do_nocb_deferred_wakeup() after the
CPU has gone completely offline.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Currently, synchronize_sched_expedited() sends IPIs to all online CPUs,
even those that are idle or executing in nohz_full= userspace. Because
idle CPUs and nohz_full= userspace CPUs are in extended quiescent states,
there is no need to IPI them in the first place. This commit therefore
avoids IPIing CPUs that are already in extended quiescent states.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There are some RCU_BOOST-specific per-CPU variable declarations that
are needlessly defined under #ifdef in kernel/rcu/tree.c. This commit
therefore moves these declarations into a pre-existing #ifdef in
kernel/rcu/tree_plugin.h.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Commit 35ce7f29a4 (rcu: Create rcuo kthreads only for onlined CPUs)
avoids creating rcuo kthreads for CPUs that never come online. This
fixes a bug in many instances of firmware: Instead of lying about their
age, these systems instead lie about the number of CPUs that they have.
Before commit 35ce7f29a4, this could result in huge numbers of useless
rcuo kthreads being created.
It appears that experience indicates that I should have told the
people suffering from this problem to fix their broken firmware, but
I instead produced what turned out to be a partial fix. The missing
piece supplied by this commit makes sure that rcu_barrier() knows not to
post callbacks for no-CBs CPUs that have not yet come online, because
otherwise rcu_barrier() will hang on systems having firmware that lies
about the number of CPUs.
It is tempting to simply have rcu_barrier() refuse to post a callback on
any no-CBs CPU that does not have an rcuo kthread. This unfortunately
does not work because rcu_barrier() is required to wait for all pending
callbacks. It is therefore required to wait even for those callbacks
that cannot possibly be invoked. Even if doing so hangs the system.
Given that posting a callback to a no-CBs CPU that does not yet have an
rcuo kthread can hang rcu_barrier(), It is tempting to report an error
in this case. Unfortunately, this will result in false positives at
boot time, when it is perfectly legal to post callbacks to the boot CPU
before the scheduler has started, in other words, before it is legal
to invoke rcu_barrier().
So this commit instead has rcu_barrier() avoid posting callbacks to
CPUs having neither rcuo kthread nor pending callbacks, and has it
complain bitterly if it finds CPUs having no rcuo kthread but some
pending callbacks. And when rcu_barrier() does find CPUs having no rcuo
kthread but pending callbacks, as noted earlier, it has no choice but
to hang indefinitely.
Reported-by: Yanko Kaneti <yaneti@declera.com>
Reported-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Reported-by: Meelis Roos <mroos@linux.ee>
Reported-by: Eric B Munson <emunson@akamai.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Eric B Munson <emunson@akamai.com>
Tested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Tested-by: Yanko Kaneti <yaneti@declera.com>
Tested-by: Kevin Fenzi <kevin@scrye.com>
Tested-by: Meelis Roos <mroos@linux.ee>
Currently, the expedited grace-period primitives do get_online_cpus().
This greatly simplifies their implementation, but means that calls
to them holding locks that are acquired by CPU-hotplug notifiers (to
say nothing of calls to these primitives from CPU-hotplug notifiers)
can deadlock. But this is starting to become inconvenient, as can be
seen here: https://lkml.org/lkml/2014/8/5/754. The problem in this
case is that some developers need to acquire a mutex from a CPU-hotplug
notifier, but also need to hold it across a synchronize_rcu_expedited().
As noted above, this currently results in deadlock.
This commit avoids the deadlock and retains the simplicity by creating
a try_get_online_cpus(), which returns false if the get_online_cpus()
reference count could not immediately be incremented. If a call to
try_get_online_cpus() returns true, the expedited primitives operate as
before. If a call returns false, the expedited primitives fall back to
normal grace-period operations. This falling back of course results in
increased grace-period latency, but only during times when CPU hotplug
operations are actually in flight. The effect should therefore be
negligible during normal operation.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Tested-by: Lan Tianyu <tianyu.lan@intel.com>
RCU currently uses for_each_possible_cpu() to spawn rcuo kthreads,
which can result in more rcuo kthreads than one would expect, for
example, derRichard reported 64 CPUs worth of rcuo kthreads on an
8-CPU image. This commit therefore creates rcuo kthreads only for
those CPUs that actually come online.
This was reported by derRichard on the OFTC IRC network.
Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Currently, RCU spawns kthreads from several different early_initcall()
functions. Although this has served RCU well for quite some time,
as more kthreads are added a more deterministic approach is required.
This commit therefore causes all of RCU's early-boot kthreads to be
spawned from a single early_initcall() function.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
The rcu_bh_qs(), rcu_preempt_qs(), and rcu_sched_qs() functions use
old-style per-CPU variable access and write to ->passed_quiesce even
if it is already set. This commit therefore updates to use the new-style
per-CPU variable access functions and avoids the spurious writes.
This commit also eliminates the "cpu" argument to these functions because
they are always invoked on the indicated CPU.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Currently TASKS_RCU would ignore a CPU running a task in nohz_full=
usermode execution. There would be neither a context switch nor a
scheduling-clock interrupt to tell TASKS_RCU that the task in question
had passed through a quiescent state. The grace period would therefore
extend indefinitely. This commit therefore makes RCU's dyntick-idle
subsystem record the task_struct structure of the task that is running
in dyntick-idle mode on each CPU. The TASKS_RCU grace period can
then access this information and record a quiescent state on
behalf of any CPU running in dyntick-idle usermode.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
RCU-tasks requires the occasional voluntary context switch
from CPU-bound in-kernel tasks. In some cases, this requires
instrumenting cond_resched(). However, there is some reluctance
to countenance unconditionally instrumenting cond_resched() (see
http://lwn.net/Articles/603252/), so this commit creates a separate
cond_resched_rcu_qs() that may be used in place of cond_resched() in
locations prone to long-duration in-kernel looping.
This commit currently instruments only RCU-tasks. Future possibilities
include also instrumenting RCU, RCU-bh, and RCU-sched in order to reduce
IPI usage.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit adds a new RCU-tasks flavor of RCU, which provides
call_rcu_tasks(). This RCU flavor's quiescent states are voluntary
context switch (not preemption!) and userspace execution (not the idle
loop -- use some sort of schedule_on_each_cpu() if you need to handle the
idle tasks. Note that unlike other RCU flavors, these quiescent states
occur in tasks, not necessarily CPUs. Includes fixes from Steven Rostedt.
This RCU flavor is assumed to have very infrequent latency-tolerant
updaters. This assumption permits significant simplifications, including
a single global callback list protected by a single global lock, along
with a single task-private linked list containing all tasks that have not
yet passed through a quiescent state. If experience shows this assumption
to be incorrect, the required additional complexity will be added.
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Currently, when RCU awakens from a wait_event_interruptible() that
might have awakened prematurely, it does a flush_signals(). This is
done on the off-chance that someone figured out how to deliver a signal
to a kthread, which is supposed to be impossible. Given that this
is supposed to be impossible, this commit changes the flush_signals()
calls into WARN_ON(signal_pending()).
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rcu_gp_kthread_wake() function checks for three conditions before
waking up grace period kthreads:
* Is the thread we are trying to wake up the current thread?
* Are the gp_flags zero? (all threads wait on non-zero gp_flags condition)
* Is there no thread created for this flavour, hence nothing to wake up?
If any one of these condition is true, we do not call wake_up().
It was found that there are quite a few avoidable wake ups both during
idle time and under stress induced by rcutorture.
Idle:
Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0
Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0
rcutorture:
Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0
Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0
Here case{1-3} are the cases listed above. We can avoid these wake
ups by using rcu_gp_kthread_wake() to conditionally wake up the grace
period kthreads.
There is a comment about an implied barrier supplied by the wake_up()
logic. This barrier is necessary for the awakened thread to see the
updated ->gp_flags. This flag is always being updated with the root node
lock held. Also, the awakened thread tries to acquire the root node lock
before reading ->gp_flags because of which there is proper ordering.
Hence this commit tries to avoid calling wake_up() whenever we can by
using rcu_gp_kthread_wake() function.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit removes a stale comment in rcu/tree.c which was left
out when some code was moved around previously in commit 2036d94a7b
("rcu: Rework detection of use of RCU by offline CPUs") For reference,
the following updated comment exists a few lines below this which means
the same:
/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Commit f7f7bac9cb ("rcu: Have the RCU tracepoints use the tracepoint_string
infrastructure") unconditionally populates the __tracepoint_str input section,
but this section is not assigned an output section if CONFIG_TRACING is not set.
This results in the __tracepoint_str turning up in unexpected places, i.e.,
after _edata.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit uses true/false instead of 1/0 for bool types in rcu_gp_fqs()
and force_qs_rnp().
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Use a bool type for return in rcu_is_watching().
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Change the remaining uses of ACCESS_ONCE() so that each ACCESS_ONCE() either does a load or a store, but not both.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The CONFIG_PROVE_RCU_DELAY Kconfig parameter doesn't appear to be very
effective at finding race conditions, so this commit removes it.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
[ paulmck: Remove definition and uses as noted by Paul Bolle. ]
Although NMI-based stack dumps are in principle more accurate, they are
also more likely to trigger deadlocks. This commit therefore replaces
all uses of trigger_all_cpu_backtrace() with rcu_dump_cpu_stacks(), so
that the CPU detecting an RCU CPU stall does the stack dumping.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
The rcu_start_future_gp() function checks the current rcu_node's ->gpnum
and ->completed twice, once without ACCESS_ONCE() and once with it.
Which is pointless because we hold that rcu_node's ->lock at that point.
The intent was to check the current rcu_node structure and the root
rcu_node structure, the latter locklessly with ACCESS_ONCE(). This
commit therefore makes that change.
The reason that it is safe to locklessly check the root rcu_nodes's
->gpnum and ->completed fields is that we hold the current rcu_node's
->lock, which constrains the root rcu_node's ability to change its
->gpnum and ->completed fields. Of course, if there is a single rcu_node
structure, then rnp_root==rnp, and holding the lock prevents all changes.
If there is more than one rcu_node structure, then the code updates the
fields in the following order:
1. Increment rnp_root->gpnum to start new grace period.
2. Increment rnp->gpnum to initialize the current rcu_node,
continuing initialization for the new grace period.
3. Increment rnp_root->completed to end the current grace period.
4. Increment rnp->completed to continue cleaning up after the
old grace period.
So there are four possible combinations of relative values of these
four fields:
N N N N: RCU idle, new grace period must be initiated.
Although rnp_root->gpnum might be incremented immediately
after we check, that will just result in unnecessary work.
The grace period already started, and we try to start it.
N+1 N N N: RCU grace period just started. No further change is
possible because we hold rnp->lock, so the checks of
rnp_root->gpnum and rnp_root->completed are stable.
We know that our request for a future grace period will
be seen during grace-period cleanup.
N+1 N N+1 N: RCU grace period is ongoing. Because rnp->gpnum is
different than rnp->completed, we won't even look at
rnp_root->gpnum and rnp_root->completed, so the possible
concurrent change to rnp_root->completed does not matter.
We know that our request for a future grace period will
be seen during grace-period cleanup, which cannot pass
this rcu_node because we hold its ->lock.
N+1 N+1 N+1 N: RCU grace period has ended, but not yet been cleaned up.
Because rnp->gpnum is different than rnp->completed, we
won't look at rnp_root->gpnum and rnp_root->completed, so
the possible concurrent change to rnp_root->completed does
not matter. We know that our request for a future grace
period will be seen during grace-period cleanup, which
cannot pass this rcu_node because we hold its ->lock.
Therefore, despite initial appearances, the lockless check is safe.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
[ paulmck: Update comment to say why the lockless check is safe. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The m68k architecture aligns only to 16-bit boundaries, which can cause
the align-to-32-bits check in __call_rcu() to trigger. Because there is
currently no known potential need for more than one low-order bit, this
commit loosens the check to 16-bit boundaries.
Reported-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
RCU contains code of the following forms:
ACCESS_ONCE(x)++;
ACCESS_ONCE(x) += y;
ACCESS_ONCE(x) -= y;
Now these constructs do operate correctly, but they really result in a
pair of volatile accesses, one to do the load and another to do the store.
This can be confusing, as the casual reader might well assume that (for
example) gcc might generate a memory-to-memory add instruction for each
of these three cases. In fact, gcc will do no such thing. Also, there
is a good chance that the kernel will move to separate load and store
variants of ACCESS_ONCE(), and constructs like the above could easily
confuse both people and scripts attempting to make that sort of change.
Finally, most of RCU's read-modify-write uses of ACCESS_ONCE() really
only need the store to be volatile, so that the read-modify-write form
might be misleading.
This commit therefore changes the above forms in RCU so that each instance
of ACCESS_ONCE() either does a load or a store, but not both. In a few
cases, ACCESS_ONCE() was not critical, for example, for maintaining
statisitics. In these cases, ACCESS_ONCE() has been dispensed with
entirely.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Those two arrays are being passed to lockdep_init_map(), which expects
const char *, and are stored in lockdep_map the same way.
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Commit ac1bea8578 (Make cond_resched() report RCU quiescent states)
fixed a problem where a CPU looping in the kernel with but one runnable
task would give RCU CPU stall warnings, even if the in-kernel loop
contained cond_resched() calls. Unfortunately, in so doing, it introduced
performance regressions in Anton Blanchard's will-it-scale "open1" test.
The problem appears to be not so much the increased cond_resched() path
length as an increase in the rate at which grace periods complete, which
increased per-update grace-period overhead.
This commit takes a different approach to fixing this bug, mainly by
moving the RCU-visible quiescent state from cond_resched() to
rcu_note_context_switch(), and by further reducing the check to a
simple non-zero test of a single per-CPU variable. However, this
approach requires that the force-quiescent-state processing send
resched IPIs to the offending CPUs. These will be sent only once
the grace period has reached an age specified by the boot/sysfs
parameter rcutree.jiffies_till_sched_qs, or once the grace period
reaches an age halfway to the point at which RCU CPU stall warnings
will be emitted, whichever comes first.
Reported-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Made rcu_momentary_dyntick_idle() as suggested by the
ktest build robot. Also fixed smp_mb() comment as noted by
Oleg Nesterov. ]
Merge with e552592e (Reduce overhead of cond_resched() checks for RCU)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The variable and struct both having the name "rcu_state" confuses
sparse in some situations, so this commit changes the variable to
"rcu_state_p" in order to avoid this confusion. This also makes
things easier for human readers.
Signed-off-by: Uma Sharma <uma.sharma523@gmail.com>
[ paulmck: Changed the declaration and several additional uses. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit allows rcutorture to print additional state for the
RCU grace-period kthreads in cases where RCU seems reluctant to
start a new grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The rcutorture output currently does not distinguish between stalls in
the RCU implementation and stalls in the rcu_torture_writer() kthreads.
This commit therefore adds some diagnostics to help distinguish between
these two conditions, at least for the non-SRCU implementations. (SRCU
does not provide evidence of update-side forward progress by design.)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
__this_cpu_ptr is being phased out.
One special case is increment_cpu_stall_ticks().
A per cpu variable is incremented so use raw_cpu_inc().
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
