Messages in this thread Patch in this message | | | Date | Sat, 30 May 2015 22:04:25 +0200 | From | Oleg Nesterov <> | Subject | ring_buffer_attach && cond_synchronize_rcu (Was: percpu-rwsem: Optimize readers and reduce global impact) |
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On 05/30, Paul E. McKenney wrote: > > But it looks like you need the RCU-sched variant. Please see below for > an untested patch providing this support. One benefit of this patch > is that it does not add any bloat to Tiny RCU.
I don't think so, see another email. But perhaps I am totally confused, please correct me.
Well, actually the first writer (need_sync == T) can use it, but it does not make sense, I think. Because it calls sync() right after it observes GP_IDLE and drops the lock, the window is too small.
> ------------------------------------------------------------------------ > > rcu: Add RCU-sched flavors of get-state and cond-sync
However, to me this patch makes sense anyway. Just I don't think rcu_sync or percpu_rw_semaphore can use the new helpers.
And. I tried to find other users of get_state/cond_sync. Found ring_buffer_attach() and it looks obviously buggy?
Again, perhaps I am totally confused, but don't we need to ensure that we have "synchronize" _between_ list_del() and list_add() ?
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?
Don't we need the patch below? (it also moves the ->rcu_pending check under "if (rb)", to make it more readable imo).
Peter?
Oleg.
--- 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); old_rb = event->rb; - event->rcu_batches = get_state_synchronize_rcu(); - event->rcu_pending = 1; - spin_lock_irqsave(&old_rb->event_lock, flags); list_del_rcu(&event->rb_entry); spin_unlock_irqrestore(&old_rb->event_lock, flags); - } - if (event->rcu_pending && rb) { - cond_synchronize_rcu(event->rcu_batches); - event->rcu_pending = 0; + 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);
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