Re: [PATCH 1/1] exit: change the release_task() paths to call flush_sigqueue() lockless

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, Feb 05, 2025 at 06:51:59PM +0100, Oleg Nesterov a écrit :
> 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).
> 
> Also, kill clear_tsk_thread_flag(TIF_SIGPENDING), it was never needed.
> 
> TODO:
> 	- we can probably shift posix_cpu_timers_exit() as well
> 	- do_sigaction() can hit the similar problem
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  kernel/exit.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 3485e5fc499e..bc2c24ea4181 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -200,20 +200,12 @@ static void __exit_signal(struct task_struct *tsk)
>  	__unhash_process(tsk, group_dead);
>  	write_sequnlock(&sig->stats_lock);
>  
> -	/*
> -	 * Do this under ->siglock, we can race with another thread
> -	 * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
> -	 */
> -	flush_sigqueue(&tsk->pending);
>  	tsk->sighand = NULL;
>  	spin_unlock(&sighand->siglock);
>  
>  	__cleanup_sighand(sighand);
> -	clear_tsk_thread_flag(tsk, TIF_SIGPENDING);

Looks good to me, except for this TIF_SIGPENDING removal which I'm less
sure about, I see a lot of places where it is added/removed. Well it's
probably only checked locally on entry code. Would it make sense to move
this chunk to a separate preceding patch? Or keep it here but at least
explain on the changelog why it is safe to remove it?

Thanks!

> -	if (group_dead) {
> -		flush_sigqueue(&sig->shared_pending);
> +	if (group_dead)
>  		tty_kref_put(tty);
> -	}
>  }
>  
>  static void delayed_put_task_struct(struct rcu_head *rhp)
> @@ -279,6 +271,11 @@ void release_task(struct task_struct *p)
>  	proc_flush_pid(thread_pid);
>  	put_pid(thread_pid);
>  	release_thread(p);
> +
> +	flush_sigqueue(&p->pending);
> +	if (thread_group_leader(p))
> +		flush_sigqueue(&p->signal->shared_pending);
> +
>  	put_task_struct_rcu_user(p);
>  
>  	p = leader;
> -- 
> 2.25.1.362.g51ebf55
> 
>