Messages in this thread | | | Date | Sat, 13 Mar 2021 17:49:36 +0100 | From | Oleg Nesterov <> | Subject | Re: [patch V2 3/3] signal: Allow tasks to cache one sigqueue struct |
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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).
> But that brings my memory back why I had cmpxchg() in the original > version. This code runs without current->sighand->siglock held.
Yes, I was wrong too. This code runs without exiting_task->sighand->siglock and this is fine in that it can not race with send_signal(exiting_task), but somehow I missed the fact that it always populates current->sigqueue_cache, not exiting_task->sigqueue_cache.
Oleg.
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