Commit 8d1f431c authored Jun 11, 2010 by Oleg Nesterov Committed by Ingo Molnar Jun 18, 2010

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



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>

parent 0bdd2ed4

kernel/posix-cpu-timers.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}