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

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Message-ID: <Z0B_38aWqStmhN24@pavilion.home>
Date: Fri, 22 Nov 2024 13:58:07 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
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

Le Fri, Nov 22, 2024 at 01:38:17PM +0100, Oleg Nesterov a écrit :
> 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.

Ok I'm good with t->exit_state, I'm cooking that.

> 
> 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;
> 

That's nice! But with the recent changes in this area, the target pick-up
logic has moved to a separate function posixtimer_get_target() which makes
this trick a bit more difficult.

I'll stick to exit_state for now.

Thanks a lot for your insight!