Re: [PATCH] [RFC] perf: Fix a race between ring_buffer_detach() and ring_buffer_wakeup()

[Peter Zijlstra]

On Thu, Mar 13, 2014 at 12:58:16PM -0700, Paul E. McKenney wrote:
> On Fri, Mar 07, 2014 at 03:38:46PM +0200, Alexander Shishkin wrote:
> > This is more of a problem description than an actual bugfix, but currently
> > ring_buffer_detach() can kick in while ring_buffer_wakeup() is traversing
> > the ring buffer's event list, leading to cpu stalls.
> > 
> > What this patch does is crude, but fixes the problem, which is: one rcu
> > grace period has to elapse between ring_buffer_detach() and subsequent
> > ring_buffer_attach(), otherwise either the attach will fail or the wakeup
> > will misbehave. Also, making it a call_rcu() callback will make it race
> > with attach().
> > 
> > Another solution that I see is to check for list_empty(&event->rb_entry)
> > before wake_up_all() in ring_buffer_wakeup() and restart the list
> > traversal if it is indeed empty, but that is ugly too as there will be
> > extra wakeups on some events.
> > 
> > Anything that I'm missing here? Any better ideas?
> 
> Not sure it qualifies as "better", but git call to ring_buffer_detach()
> is going to free the event anyway, so the synchronize_rcu() and the
> INIT_LIST_HEAD() should not be needed in that case.  I am guessing that
> the same is true for perf_mmap_close().
> 
> So that leaves the call in perf_event_set_output(), which detaches from an
> old rb before attaching that same event to a new one.  So maybe have the
> synchronize_rcu() and INIT_LIST_HEAD() instead be in the "if (old_rb)",
> which might be a reasonably uncommon case?

How about something like so that only does the sync_rcu() if really
needed.


---
 kernel/events/core.c     | 11 +++++++++--
 kernel/events/internal.h |  1 +
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 661951ab8ae7..88c8c810e081 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3856,12 +3856,17 @@ static void ring_buffer_attach(struct perf_event *event,
 {
 	unsigned long flags;
 
+	if (rb->rcu_batches == rcu_batches_completed()) {
+		synchronize_rcu();
+		INIT_LIST_HEAD(&event->rb_entry);
+	}
+
 	if (!list_empty(&event->rb_entry))
 		return;
 
 	spin_lock_irqsave(&rb->event_lock, flags);
 	if (list_empty(&event->rb_entry))
-		list_add(&event->rb_entry, &rb->event_list);
+		list_add_rcu(&event->rb_entry, &rb->event_list);
 	spin_unlock_irqrestore(&rb->event_lock, flags);
 }
 
@@ -3873,9 +3878,11 @@ static void ring_buffer_detach(struct perf_event *event, struct ring_buffer *rb)
 		return;
 
 	spin_lock_irqsave(&rb->event_lock, flags);
-	list_del_init(&event->rb_entry);
+	list_del_rcu(&event->rb_entry);
 	wake_up_all(&event->waitq);
 	spin_unlock_irqrestore(&rb->event_lock, flags);
+
+	rb->rcu_batches = rcu_batches_completed();
 }
 
 static void ring_buffer_wakeup(struct perf_event *event)
diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 569b218782ad..698b5881b2a4 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -30,6 +30,7 @@ struct ring_buffer {
 	/* poll crap */
 	spinlock_t			event_lock;
 	struct list_head		event_list;
+	unsigned long			rcu_batches;
 
 	atomic_t			mmap_count;
 	unsigned long			mmap_locked;
--
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