Messages in this thread | | | Date | Thu, 14 Apr 2011 21:00:12 +0200 | From | Oleg Nesterov <> | Subject | Re: [RFC][PATCH 2/5] signals: Introduce per-thread siglock and action rwlock |
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On 04/14, Matt Fleming wrote: > > Well, it's not really that signals are slow (though that might be > true!), it's more that they don't scale. So, this patch series was not > designed to speed up signals in a single-threaded app, but rather to > make signals scale better in multithreaded apps. We do that by > reducing the contention on the shared siglock. Signal delivery isn't > getting any faster with these patches, they just try to stop it getting > slower when you add more threads ;-)
Yes, I understand. Even the private signal needs the "global" per-process lock bit it is rwlock.
> > > @@ -142,7 +142,9 @@ static void __exit_signal(struct task_struct *tsk) > > > * Do this under ->siglock, we can race with another thread > > > * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals. > > > */ > > > + spin_lock(&tsk->siglock); > > > flush_sigqueue(&tsk->pending); > > > + spin_unlock(&tsk->siglock); > > > > This only protects flush_sigqueue(), but this is not enough. > > > > tkill() can run without ->siglock held, how can it ensure the target > > can't exit before we take task->siglock? > > And by "target can't exit" you mean, how can we ensure that the target > doesn't execute __exit_signal() and set tsk->signal to NULL
No, tsk->signal can't go away, ->sighand can. This means that prepare_signal()->sig_ignored() is not safe.
Another problem is, __send_signal() shouldn't add the new sigqueue to tsk->pending if this tsk has already passed __exit_signal()->flush_sigqueue().
> While we're discussing the technique for stopping tasks from exiting... > > Is there a reason that a short-term reference counter isn't used to > prevent this, instead of taking the siglock?
Well, sighand->count is the reference counter. The problem is, ->sighand is not per-process, we can share it with abother CLONE_SIGHAND process and de_thread() can change ->sighand during exec.
Also. We have the code which checks ->sighand != NULL to check if this thread was released or not.
I was going to try to add some cleanups here after the scope of ->signal was changed, but right now I can't recall what I had in mind. Anyway, everything in sighand_struct needs ->siglock anyway, a lockless access doesn't buy too much currently.
> > > @@ -1666,7 +1779,8 @@ static void do_notify_parent_cldstop(struct task_struct *tsk, > > > } > > > > > > sighand = parent->sighand; > > > - spin_lock_irqsave(&sighand->siglock, flags); > > > + read_lock_irqsave(&sighand->action_lock, flags); > > > + spin_lock(&sighand->siglock); > > > > Why do we need both? (the same for do_notify_parent) > > We need action_lock because we're reading sighand->action and making > decisions based upon its value, so we need it to not change. Also, > __send_signal()
Ah, indeed, thanks. Somehow I misread the code as if it takes task->siglock, not sighand->siglock. But anyway I was wrong, I forgot we are going to send the signal.
Oleg.
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