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sched: Fix the racy usage of thread_group_cputimer() in fastpath_timer_check()

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fastpath_timer_check()->thread_group_cputimer() is racy and
unneeded.

It is racy because another thread can clear ->running before
thread_group_cputimer() takes cputimer->lock. In this case
thread_group_cputimer() will set ->running = true again and call
thread_group_cputime(). But since we do not hold tasklist or
siglock, we can race with fork/exit and copy the wrong results
into cputimer->cputime.

It is unneeded because if ->running == true we can just use
the numbers in cputimer->cputime we already have.

Change fastpath_timer_check() to copy cputimer->cputime into
the local variable under cputimer->lock. We do not re-check
->running under cputimer->lock, run_posix_cpu_timers() does
this check later.

Note: we can add more optimizations on top of this change.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100611180446.GA13025@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Oleg Nesterov 2010-06-11 20:04:46 +02:00 committed by Ingo Molnar
parent 0bdd2ed413
commit 8d1f431cbe
1 changed files with 4 additions and 1 deletions
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5
kernel/posix-cpu-timers.c
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@ -1287,7 +1287,10 @@ static inline int fastpath_timer_check(struct task_struct *tsk)
if (sig->cputimer.running) {
struct task_cputime group_sample;
thread_group_cputimer(tsk, &group_sample);
spin_lock(&sig->cputimer.lock);
group_sample = sig->cputimer.cputime;
spin_unlock(&sig->cputimer.lock);
if (task_cputime_expired(&group_sample, &sig->cputime_expires))
return 1;
}