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timers: Plug locking race vs. timer migration

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commit b831275a3553c32091222ac619cfddd73a5553fb upstream.

Linus noticed that lock_timer_base() lacks a READ_ONCE() for accessing the
timer flags. As a consequence the compiler is allowed to reload the flags
between the initial check for TIMER_MIGRATION and the following timer base
computation and the spin lock of the base.

While this has not been observed (yet), we need to make sure that it never
happens.

Fixes: 0eeda71bc30d ("timer: Replace timer base by a cpu index")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1610241711220.4983@nanos
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Thomas Gleixner 2016-10-24 11:41:56 +02:00 committed by Greg Kroah-Hartman
parent 08b1cf4964
commit 839003061a
1 changed files with 8 additions and 1 deletions
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9
kernel/time/timer.c
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@ -764,8 +764,15 @@ static struct tvec_base *lock_timer_base(struct timer_list *timer,
__acquires(timer->base->lock)
{
for (;;) {
u32 tf = timer->flags;
struct tvec_base *base;
u32 tf;
/*
* We need to use READ_ONCE() here, otherwise the compiler
* might re-read @tf between the check for TIMER_MIGRATING
* and spin_lock().
*/
tf = READ_ONCE(timer->flags);
if (!(tf & TIMER_MIGRATING)) {
base = per_cpu_ptr(&tvec_bases, tf & TIMER_CPUMASK);