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Subject: Re: [PATCH] fix send_sigqueue() vs thread exit race
From: Thomas Gleixner <tglx@linutronix.de>
Reply-To: tglx@linutronix.de
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Ingo Molnar <mingo@elte.hu>, Roland McGrath <roland@redhat.com>,
       George Anzinger <george@mvista.com>, linux-kernel@vger.kernel.org,
       Steven Rostedt <rostedt@goodmis.org>,
       "Paul E. McKenney" <paulmck@us.ibm.com>, Andrew Morton <akpm@osdl.org>
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Organization: linutronix
Date: Sun, 21 Aug 2005 23:50:32 +0200
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On Sun, 2005-08-21 at 23:24 +0200, Thomas Gleixner wrote:
> Oleg,

Sorry for making noise. I introduced the race again.

tglx


--- 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 23:48:52.000000000 +0200
@@ -1381,8 +1381,15 @@ send_sigqueue(int sig, struct sigqueue *
 	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;
+	}
 	if (unlikely(p->flags & PF_EXITING)) {
 		ret = -1;
 		goto out_err;
@@ -1427,7 +1434,18 @@ 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;
+	}
+	if (unlikely(p->flags & PF_EXITING)) {
+		ret = -1;
+		goto out_err;
+	}
 	spin_lock_irqsave(&p->sighand->siglock, flags);
 	handle_stop_signal(sig, p);
 
@@ -1461,8 +1479,9 @@ send_group_sigqueue(int sig, struct sigq
 	__group_complete_signal(sig, p);
 out:
 	spin_unlock_irqrestore(&p->sighand->siglock, flags);
+out_err:
 	read_unlock(&tasklist_lock);
-	return(ret);
+	return ret;
 }
 
 /*
--- 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