linux-stable.git/kernel/rcu, branch v3.18.136

rcu: Clear need_qs flag to prevent splat

2018-11-10T15:39:17+00:00

[ Upstream commit c0135d07b013fa8f7ba9ec91b4369c372e6a28cb ]

If the scheduling-clock interrupt sets the current tasks need_qs flag,
but if the current CPU passes through a quiescent state in the meantime,
then rcu_preempt_qs() will fail to clear the need_qs flag, which can fool
RCU into thinking that additional rcu_read_unlock_special() processing
is needed.  This commit therefore clears the need_qs flag before checking
for additional processing.

For this problem to occur, we need rcu_preempt_data.passed_quiesce equal
to true and current->rcu_read_unlock_special.b.need_qs also equal to true.
This condition can occur as follows:

1.	CPU 0 is aware of the current preemptible RCU grace period,
	but has not yet passed through a quiescent state.  Among other
	things, this means that rcu_preempt_data.passed_quiesce is false.

2.	Task A running on CPU 0 enters a preemptible RCU read-side
	critical section.

3.	CPU 0 takes a scheduling-clock interrupt, which notices the
	RCU read-side critical section and the need for a quiescent state,
	and thus sets current->rcu_read_unlock_special.b.need_qs to true.

4.	Task A is preempted, enters the scheduler, eventually invoking
	rcu_preempt_note_context_switch() which in turn invokes
	rcu_preempt_qs().

	Because rcu_preempt_data.passed_quiesce is false,
	control enters the body of the "if" statement, which sets
	rcu_preempt_data.passed_quiesce to true.

5.	At this point, CPU 0 takes an interrupt.  The interrupt
	handler contains an RCU read-side critical section, and
	the rcu_read_unlock() notes that current->rcu_read_unlock_special
	is nonzero, and thus invokes rcu_read_unlock_special().

6.	Once in rcu_read_unlock_special(), the fact that
	current->rcu_read_unlock_special.b.need_qs is true becomes
	apparent, so rcu_read_unlock_special() invokes rcu_preempt_qs().
	Recursively, given that we interrupted out of that same
	function in the preceding step.

7.	Because rcu_preempt_data.passed_quiesce is now true,
	rcu_preempt_qs() does nothing, and simply returns.

8.	Upon return to rcu_read_unlock_special(), it is noted that
	current->rcu_read_unlock_special is still nonzero (because
	the interrupted rcu_preempt_qs() had not yet gotten around
	to clearing current->rcu_read_unlock_special.b.need_qs).

9.	Execution proceeds to the WARN_ON_ONCE(), which notes that
	we are in an interrupt handler and thus duly splats.

The solution, as noted above, is to make rcu_read_unlock_special()
clear out current->rcu_read_unlock_special.b.need_qs after calling
rcu_preempt_qs().  The interrupted rcu_preempt_qs() will clear it again,
but this is harmless.  The worst that happens is that we clobber another
attempt to set this field, but this is not a problem because we just
got done reporting a quiescent state.

Reported-by: Sasha Levin 
Signed-off-by: Paul E. McKenney 
[ paulmck: Fix embarrassing build bug noted by Sasha Levin. ]
Tested-by: Sasha Levin 
Signed-off-by: Sasha Levin

rcu: Correctly handle non-empty Tiny RCU callback list with none ready

2015-07-04T03:02:21+00:00

[ Upstream commit 6e91f8cb138625be96070b778d9ba71ce520ea7e ]

If, at the time __rcu_process_callbacks() is invoked,  there are callbacks
in Tiny RCU's callback list, but none of them are ready to be invoked,
the current list-management code will knit the non-ready callbacks out
of the list.  This can result in hangs and possibly worse.  This commit
therefore inserts a check for there being no callbacks that can be
invoked immediately.

This bug is unlikely to occur -- you have to get a new callback between
the time rcu_sched_qs() or rcu_bh_qs() was called, but before we get to
__rcu_process_callbacks().  It was detected by the addition of RCU-bh
testing to rcutorture, which in turn was instigated by Iftekhar Ahmed's
mutation testing.  Although this bug was made much more likely by
915e8a4fe45e (rcu: Remove fastpath from __rcu_process_callbacks()), this
did not cause the bug, but rather made it much more probable.   That
said, it takes more than 40 hours of rcutorture testing, on average,
for this bug to appear, so this fix cannot be considered an emergency.

Signed-off-by: Paul E. McKenney 
Cc: 
Reviewed-by: Josh Triplett 
Signed-off-by: Sasha Levin

rcu: Make rcu_barrier() understand about missing rcuo kthreads

2014-10-28T20:24:13+00:00

Commit 35ce7f29a44a (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 35ce7f29a44a, 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 
Reported-by: Jay Vosburgh 
Reported-by: Meelis Roos 
Reported-by: Eric B Munson 
Signed-off-by: Paul E. McKenney 
Tested-by: Eric B Munson 
Tested-by: Jay Vosburgh 
Tested-by: Yanko Kaneti 
Tested-by: Kevin Fenzi 
Tested-by: Meelis Roos

rcu: Eliminate deadlock between CPU hotplug and expedited grace periods

2014-09-18T23:22:27+00:00

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 
Cc: Josh Triplett 
Cc: "Rafael J. Wysocki" 
Tested-by: Lan Tianyu

rcutorture: Rename rcutorture_runnable parameter

2014-09-16T20:41:44+00:00

This commit changes rcutorture_runnable to torture_runnable, which is
consistent with the names of the other parameters and is a bit shorter
as well.

Signed-off-by: Paul E. McKenney

torture: Address race in module cleanup

2014-09-16T20:41:06+00:00

When performing module cleanups by calling torture_cleanup() the
'torture_type' string in nullified However, callers are not necessarily
done, and might still need to reference the variable. This impacts
both rcutorture and locktorture, causing printing things like:

[   94.226618] (null)-torture: Stopping lock_torture_writer task
[   94.226624] (null)-torture: Stopping lock_torture_stats task

Thus delay this operation until the very end of the cleanup process.
The consequence (which shouldn't matter for this kid of program) is,
of course, that we delay the window between rmmod and modprobing,
for instance in module_torture_begin().

Signed-off-by: Davidlohr Bueso 
Signed-off-by: Paul E. McKenney

Merge branch 'rcu-tasks.2014.09.10a' into HEAD

2014-09-16T17:10:44+00:00

rcu-tasks.2014.09.10a: Add RCU-tasks flavor of RCU.

Merge branches 'doc.2014.09.07a', 'fixes.2014.09.10a', 'nocb-nohz.2014.09.16b' and 'torture.2014.09.07a' into HEAD

2014-09-16T17:08:34+00:00

doc.2014.09.07a: Documentation updates.
fixes.2014.09.10a: Miscellaneous fixes.
nocb-nohz.2014.09.16b: No-CBs CPUs and NO_HZ_FULL updates.
torture.2014.09.07a: Torture-test updates.

rcu: Avoid misordering in nocb_leader_wait()

2014-09-16T17:08:03+00:00

The NOCB follower wakeup ordering depends on the store to the tail
pointer happening before the wakeup.  However, because atomic_long_add()
does not return a value, it does not provide ordering guarantees, and
the locking in wake_up() only guarantees that the store will happen
before the unlock, which might be too late.  Even though this is only a
theoretical issue, this commit adds a smp_mb__after_atomic() after the
final atomic_long_add() to provide the needed ordering guarantee.

Reported-by: Amit Shah 
Signed-off-by: Paul E. McKenney 
Tested-by: Paul Gortmaker

rcu: Handle NOCB callbacks from irq-disabled idle code

2014-09-16T17:08:03+00:00

If an RCU callback is queued on a no-CBs CPU from idle code with irqs
disabled, and if that CPU stays idle forever after, the callback will
never be invoked.  This commit therefore adds a check for this situation
in ____call_rcu_nocb(), invoking the RCU core solely for the purpose
of the ensuing return-to-idle transition.  (If the CPU doesn't return
to idle, the next scheduling-clock interrupt will fix things up.)

Reported-by: Amit Shah 
Signed-off-by: Paul E. McKenney 
Tested-by: Paul Gortmaker