On 08/24, Sukadev Bhattiprolu wrote: > > Oleg Nesterov wrote: > >On 08/24, taoyue wrote: > > > >>Oleg Nesterov wrote: > >> > >>>>collect_signal: sigqueue_free: > >>>> > >>>> list_del_init(&first->list); > >>>> spin_lock_irqsave(lock, flags); > >>>> > >>>> > >>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > >>> > >>> > >>>> if (!list_empty(&q->list)) > >>>> list_del_init(&q->list); > >>>> spin_unlock_irqrestore(lock, > >>>> flags); > >>>> q->flags &= ~SIGQUEUE_PREALLOC; > >>>> > >>>> __sigqueue_free(first); __sigqueue_free(q); > >>>> > >>>> > >>>collect_signal() is always called under ->siglock which is also taken by > >>>sigqueue_free(), so this is not possible. > >>> > >>> > >>> > >>I know, using current->sighand->siglock to prevent one sigqueue > >>is free twice. I want to know whether it is possible that the two > >>function is called in different thread. If that, the spin_lock is useless. > >> > > > >Not sure I understand. Yes, it is possible they are called by 2 different > >threads, that is why we had a race. But all threads in the same thread > >group have the same ->sighand, and thus the same ->sighand->siglock. > > > > Oleg, if one thread can be in collect_signal() and another in > sigqueue_free() and both operate on the exact same sigqueue object, its > not clear how we prevent two calls to __sigqueue_free() to > the same object. In that case the lock (or some lock) should be around > __sigqueue_free() - no ? > > i.e if we enter sigqueue_free(), we will call __sigqueue_free() > regardless of the state.
Yes. They both will call __sigqueue_free(). But please note that __sigqueue_free() checks SIGQUEUE_PREALLOC, which is cleared by sigqueue_free(). IOW, when sigqueue_free() unlocks ->siglock, we know that it can't be used by collect_signal() from another thread. So we can clear SIGQUEUE_PREALLOC and free sigqueue. We don't need this lock around sigqueue_free() to prevent the race. collect_signal() can "see" only those sigqueues which are on list. IOW, when sigqueue_free() takes ->siglock, colect_signal() can't run, because it needs the same lock. Now we delete this sigqueue from list, nobody can see it, it can't have other references. So we can unlock ->siglock, mark sigqueue as freeable (clear SIGQUEUE_PREALLOC), and free it. Do you agree? Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
