linux-kernel - Re: posix timer freeze after some random time, under pthread create/destroy load

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Message-ID: <20241122123817.GC24815@redhat.com>
Date: Fri, 22 Nov 2024 13:38:17 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Anthony Mallet <anthony.mallet@...s.fr>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: posix timer freeze after some random time, under pthread
 create/destroy load

On 11/22, Frederic Weisbecker wrote:
>
> Le Fri, Nov 22, 2024 at 12:49:50PM +0100, Oleg Nesterov a écrit :
> > On 11/22, Frederic Weisbecker wrote:
> > >
> > >
> > > Right, I don't mind either way,
> >
> > Me too, so feel free to ignore,
> >
> > > though if it's past PF_EXITING,
> > > complete_signal() -> wants_signal() will defer to another thread anyway, right?
> >
> > Right. So I think it would be better to rely on complete_signal() in this
> > case even if the current logic is very simple and dumb.
>
> Just to make sure I understand correctly, this means you'd prefer to keep
> the PF_EXITING test?

No, sorry for confusion ;)

I'd prefer to check t->exit_state in send_sigqueue() and let complete_signal()
pick another thread if "t->flags & PF_EXITING" is already set.

But I am fine either way, up to you.

I guess we can even avoid the additional check altogether, something like below.
Again, up to you. Your approach looks simpler and doesn't need more comments.

Oleg.

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1966,7 +1966,7 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
 {
 	int sig = q->info.si_signo;
 	struct sigpending *pending;
-	struct task_struct *t;
+	struct task_struct *g, *t;
 	unsigned long flags;
 	int ret, result;
 
@@ -1989,12 +1989,12 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
 	 * the same thread group as the target process, which avoids
 	 * unnecessarily waking up a potentially idle task.
 	 */
-	t = pid_task(pid, type);
-	if (!t)
+	g = t = pid_task(pid, type);
+	if (!g)
 		goto ret;
 	if (type != PIDTYPE_PID && same_thread_group(t, current))
 		t = current;
-	if (!likely(lock_task_sighand(t, &flags)))
+	if (!likely(lock_task_sighand(g, &flags)))
 		goto ret;
 
 	ret = 1; /* the signal is ignored */
@@ -2022,7 +2022,7 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
 	result = TRACE_SIGNAL_DELIVERED;
 out:
 	trace_signal_generate(sig, &q->info, t, type != PIDTYPE_PID, result);
-	unlock_task_sighand(t, &flags);
+	unlock_task_sighand(g, &flags);
 ret:
 	rcu_read_unlock();
 	return ret;