commit 8d1f431cbec115a780cd551ab1b4955c125f8d31
author Oleg Nesterov <oleg@redhat.com> Fri Jun 11 20:04:46 2010 +0200
committer Ingo Molnar <mingo@elte.hu> Fri Jun 18 10:46:57 2010 +0200
tree f71e707f0ff495582bd2f43017c87b2ac6ec519d
parent 0bdd2ed4138ec04e09b4f8165981efc99e439f55

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>