Messages in this thread | | | Date | Tue, 16 Jun 2015 13:08:55 +0200 | From | Peter Zijlstra <> | Subject | Re: ring_buffer_attach && cond_synchronize_rcu (Was: percpu-rwsem: Optimize readers and reduce global impact) |
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On Sat, May 30, 2015 at 10:04:25PM +0200, Oleg Nesterov wrote: > And. I tried to find other users of get_state/cond_sync. Found > ring_buffer_attach() and it looks obviously buggy?
Urgh, indeed.
> IOW. Suppose that ring_buffer_attach() preempts right_after > get_state_synchronize_rcu() and gp completes before spin_lock(). > > In this case cond_synchronize_rcu() does nothing and we reuse > ->rb_entry without waiting for gp in between?
Yes.
> Don't we need the patch below? (it also moves the ->rcu_pending check > under "if (rb)", to make it more readable imo).
> --- x/kernel/events/core.c > +++ x/kernel/events/core.c > @@ -4310,20 +4310,20 @@ static void ring_buffer_attach(struct pe > WARN_ON_ONCE(event->rcu_pending); > > spin_lock_irqsave(&old_rb->event_lock, flags); > list_del_rcu(&event->rb_entry); > spin_unlock_irqrestore(&old_rb->event_lock, flags); > > + event->rcu_batches = get_state_synchronize_rcu(); > + event->rcu_pending = 1; > } > > if (rb) { > + if (event->rcu_pending) { > + cond_synchronize_rcu(event->rcu_batches); > + event->rcu_pending = 0; > + } > + > spin_lock_irqsave(&rb->event_lock, flags); > list_add_rcu(&event->rb_entry, &rb->event_list); > spin_unlock_irqrestore(&rb->event_lock, flags);
Agreed.
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