Re: [patch V2 3/3] signal: Allow tasks to cache one sigqueue struct

[Thomas Gleixner <tglx@linutronix.de>]

On Sat, Mar 13 2021 at 17:49, Oleg Nesterov wrote:
> On 03/12, Thomas Gleixner wrote:
>>
>> On Fri, Mar 12 2021 at 20:26, Thomas Gleixner wrote:
>> > On Fri, Mar 12 2021 at 17:11, Oleg Nesterov wrote:
>> >> On 03/11, Thomas Gleixner wrote:
>> >>>
>> >>> @@ -456,7 +460,12 @@ static void __sigqueue_free(struct sigqu
>> >>>  		return;
>> >>>  	if (atomic_dec_and_test(&q->user->sigpending))
>> >>>  		free_uid(q->user);
>> >>> -	kmem_cache_free(sigqueue_cachep, q);
>> >>> +
>> >>> +	/* Cache one sigqueue per task */
>> >>> +	if (!current->sigqueue_cache)
>> >>> +		current->sigqueue_cache = q;
>> >>> +	else
>> >>> +		kmem_cache_free(sigqueue_cachep, q);
>> >>>  }
>> >>
>> >> This doesn't look right, note that __exit_signal() does
>> >> flush_sigqueue(&sig->shared_pending) at the end, after exit_task_sighand()
>> >> was already called.
>> >>
>> >> I'd suggest to not add the new exit_task_sighand() helper and simply free
>> >> current->sigqueue_cache at the end of __exit_signal().
>> >
>> > Ooops. Thanks for spotting this!
>>
>> Hrm.
>>
>> The task which is released is obviously not current, so even if there
>> are still sigqueues in shared_pending then they wont end up in the
>> released tasks sigqueue_cache. They can only ever end up in
>> current->sigqueue_cache.
>
> The task which is released can be "current" if this task autoreaps.
> For example, if its parent ignores SIGCHLD. In this case exit_notify()
> does release_task(current).

Bah. Let me stare at it moar.