linux-kernel - Re: [PATCH 1/1] exit: change the release_task() paths to call flush

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Message-ID: <874j17nk11.ffs@tglx>
Date: Thu, 06 Feb 2025 16:57:14 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Oleg Nesterov <oleg@...hat.com>, Andrew Morton
 <akpm@...ux-foundation.org>, "Eric W. Biederman" <ebiederm@...ssion.com>,
 Frederic Weisbecker <frederic@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>
Cc: Mateusz Guzik <mjguzik@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] exit: change the release_task() paths to call
 flush_sigqueue() lockless

On Wed, Feb 05 2025 at 18:51, Oleg Nesterov wrote:
> A task can block a signal, accumulate up to RLIMIT_SIGPENDING sigqueues,
> and exit. In this case __exit_signal()->flush_sigqueue() called with irqs
> disabled can triger a hard lockup, see
> https://lore.kernel.org/all/20190322114917.GC28876@redhat.com/
>
> Fortunately, after the recent posixtimer changes sys_timer_delete() paths
> no longer try to clear SIGQUEUE_PREALLOC and/or free tmr->sigq, and after
> the exiting task passes __exit_signal() lock_task_sighand() can't succeed
> and pid_task(tmr->it_pid) will return NULL.
>
> This means that after __exit_signal(tsk) nobody can play with tsk->pending
> or (if group_dead) with tsk->signal->shared_pending, so release_task() can
> safely call flush_sigqueue() after write_unlock_irq(&tasklist_lock).

I can't find a problem with that.

> Also, kill clear_tsk_thread_flag(TIF_SIGPENDING), it was never needed.

I'm not entirely sure about that, but it does not hurt to clear it,
right?

> TODO:
> 	- we can probably shift posix_cpu_timers_exit() as well

I think so.

> 	- do_sigaction() can hit the similar problem

Indeed, but that's a tough on to solve.

Thanks,

        tglx