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SubjectRe: ring_buffer_attach && cond_synchronize_rcu (Was: percpu-rwsem: Optimize readers and reduce global impact)
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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