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

[Oleg Nesterov]

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@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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