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SubjectRe: [PATCH] fix send_sigqueue() vs thread exit race
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Oleg,

On Sun, 2005-08-21 at 14:59 +0400, Oleg Nesterov wrote:
> CPU_0 CPU_1
>
> release_task: posix_timer_fn:
> write_lock(tasklist); lock(timer->it_lock);
>
> exit_timers: send_sigxxx();
> lock(timer->it_lock) read_lock(tasklist);
>
> Deadlock.
>
> Dear cc-list, what do you think?

The patch below on top of your patch should solve this. We don't need
tasklist_lock to check p->flags. As you pointed out p cannot be invalid
in send_sigqueue as it's protected by get_task_struct() in
create_timer()

For send_group_sigqueue it's protected by exit_itimers() waiting for
k_itimer.it_lock.

It still does not solve the ugly dependency on tasklist_lock but at
least the race and the deadlock are fixed.

tglx

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>


--- linux-2.6.13-rc6.signal/kernel/signal.oleg.c 2005-08-21 23:07:03.000000000 +0200
+++ linux-2.6.13-rc6.signal/kernel/signal.c 2005-08-21 23:09:07.000000000 +0200
@@ -1381,11 +1381,14 @@ send_sigqueue(int sig, struct sigqueue *
int ret = 0;

BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
- read_lock(&tasklist_lock);

- if (unlikely(p->flags & PF_EXITING)) {
- ret = -1;
- goto out_err;
+retry:
+ if (unlikely(p->flags & PF_EXITING))
+ return -1;
+
+ if ((unlikely(!read_trylock(&tasklist_lock))) {
+ cpu_relax();
+ goto retry;
}

spin_lock_irqsave(&p->sighand->siglock, flags);
@@ -1414,7 +1417,6 @@ send_sigqueue(int sig, struct sigqueue *

out:
spin_unlock_irqrestore(&p->sighand->siglock, flags);
-out_err:
read_unlock(&tasklist_lock);

return ret;
@@ -1427,7 +1429,14 @@ send_group_sigqueue(int sig, struct sigq
int ret = 0;

BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
- read_lock(&tasklist_lock);
+retry:
+ if (unlikely(p->flags & PF_EXITING))
+ return -1;
+
+ if (unlikely(!read_trylock(&tasklist_lock))) {
+ cpu_relax();
+ goto retry;
+ }
spin_lock_irqsave(&p->sighand->siglock, flags);
handle_stop_signal(sig, p);

--- linux-2.6.13-rc6.signal/kernel/posix-timers.oleg.c 2005-08-21 23:09:58.000000000 +0200
+++ linux-2.6.13-rc6.signal/kernel/posix-timers.c 2005-08-21 23:19:42.000000000 +0200
@@ -501,7 +501,8 @@ static void posix_timer_fn(unsigned long
remove_from_abslist(timr);
}

- if (posix_timer_event(timr, si_private))
+ /* Do not rearm the timer, when we are exiting */
+ if (posix_timer_event(timr, si_private) > 0)
/*
* signal was not sent because of sig_ignor
* we will not get a call back to restart it AND

-
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