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

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Message-ID: <20241122114949.GA24815@redhat.com>
Date: Fri, 22 Nov 2024 12:49:50 +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 09:24:07AM +0100, Oleg Nesterov a écrit :
> > On 11/21, Frederic Weisbecker wrote:
> > >
> > > I think this started with commit:
> > >
> > > bcb7ee79029d (posix-timers: Prefer delivery of signals to the current thread)
> > >
> > > The problem is that if the current task is exiting and has already been reaped,
> > > its sighand pointer isn't there anymore.
> >
> > Thanks...
> >
> > This can only happen if the exiting task has already passed exit_notify() which
> > sets exit_state. So I'd suggest to check current->exit_state instead of PF_EXITING
> > in the patch below.
> >
> > Oleg.
>
> 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.

> Due to retarget_shared_pending() being called after the flag being set...

Yes. Whatever we do send_sigqueue/complete_signal can choose an exiting thread
which doesn't have PF_EXITING yet, in this case retarget_shared_pending() from
that thread will pick another target for signal_wake_up/TIF_SIGPENDING.

Thanks!

Oleg.