Re: [PATCH 6/5] fix the racy usage of thread_group_cputimer() infastpath_timer_check()

From: Oleg Nesterov
Date: Fri Jun 11 2010 - 14:07:45 EST


On 06/11, Oleg Nesterov wrote:
>
> fastpath_timer_check()->thread_group_cputimer() is racy and
> unneeded.

Just in case... this fix doesn't depend on other patches I sent.

> 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@xxxxxxxxxx>
> ---
>
> kernel/posix-cpu-timers.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> --- 35-rc2/kernel/posix-cpu-timers.c~6_FPTC_DONT_SET_RUNNING 2010-06-11 01:08:03.000000000 +0200
> +++ 35-rc2/kernel/posix-cpu-timers.c 2010-06-11 19:40:22.000000000 +0200
> @@ -1287,7 +1287,10 @@ static inline int fastpath_timer_check(s
> 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;
> }

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