Re: [PATCH] fix send_sigqueue() vs thread exit race

[Oleg Nesterov <oleg@tv-sign.ru>]

Thomas Gleixner wrote:
>
> It exists. It triggers on preempt-RT and I can trigger it on vanilla SMP
> by waiting for the timer expiry in release_task() before the
> __exit_signal() call. That's reasonable, as it can happen that way by
> chance too. It requires that the timer expires on a different CPU, but I
> don't see a reason why this should not happen.

Ok, exit_itimers()->itimer_delete() called when the last thread exits
or does exec.

kernel/posix-timers.c:common_timer_del() calls del_timer_sync(), after
that nobody can access this timer, so we don't need to lock timer->it_lock
at all in this case. No lock - no deadlock.

But I know nothing about kernel/posix-cpu-timers.c, I doubt it will work
for posix_cpu_timer_del(). I don't have time to study posix-cpu-timers now.
However, I see that __exit_signal() calls posix_cpu_timers_exit_xxx(), so
may be it can work?

   380  int posix_cpu_timer_del(struct k_itimer *timer)
   381  {
   382          struct task_struct *p = timer->it.cpu.task;
   383
   384          if (timer->it.cpu.firing)
   385                  return TIMER_RETRY;
   386
   387          if (unlikely(p == NULL))
   388                  return 0;
   389
   390          if (!list_empty(&timer->it.cpu.entry)) {
   391                  read_lock(&tasklist_lock);

Surely, it should be impossible to happen when process exists, otherwise
it would deadlock immediately, we did write_lock(tasklist).

Thomas, do you know something about posix-cpu-timers.c?

Oleg.
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