Re: [PATCH] signal: Use list_del_init_careful() in flush_sigqueue()

[Thomas Gleixner <tglx@kernel.org>]

On Mon, Aug 24 2026 at 15:59, Frederic Weisbecker wrote:
> Le Mon, Aug 24, 2026 at 01:54:26PM +0200, Oleg Nesterov a écrit :
>> > Hmm, at first glance... If we change de_thread() to do this _after_ transfer_pid's
>> > (before release_task(leader)), then posixtimer_send_sigqueue() doesn't need any
>> > changes, no?
>> 
>> IOW. Unless I am totally confused, we only need to flush the
>> SIGQUEUE_PREALLOC sigqueue's which were sent to the (old) leader
>> before it changed its pid. So we can do this
>> 
>> diff --git a/fs/exec.c b/fs/exec.c
>> index a14f28b15607..550367e7fe6c 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1029,6 +1029,9 @@ static int de_thread(struct task_struct *tsk)
>>  		write_unlock_irq(&tasklist_lock);
>>  		cgroup_threadgroup_change_end(tsk);
>>  
>> +		scoped_guard(spinlock_irq, lock)
>> +			flush_sigqueue(&leader->pending);
>> +
>
> Is there something to prevent the timer from firing on another CPU,
> racing with this tiny window and queue the signal to the old leader? After
> all exchange_tids() is just some RCU pointers changed but there is nothing
> to synchronize the readers before the flush_sigqueue(). So pid_task() may
> still return the old leader after it?

You beat me to it.

That's what I initialy thought when I added that exiting check into
posixtimer_send_queue(), but then the trivial variant lured me away. :)

Let me go and polish up that initial variant and write a change log.

Thanks,

        tglx