From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755175AbaCMT6Y (ORCPT ); Thu, 13 Mar 2014 15:58:24 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:37466 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754740AbaCMT6V (ORCPT ); Thu, 13 Mar 2014 15:58:21 -0400 Date: Thu, 13 Mar 2014 12:58:16 -0700 From: "Paul E. McKenney" To: Alexander Shishkin Cc: Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, Frederic Weisbecker , Mike Galbraith , Paul Mackerras , Stephane Eranian , Andi Kleen Subject: Re: [PATCH] [RFC] perf: Fix a race between ring_buffer_detach() and ring_buffer_wakeup() Message-ID: <20140313195816.GJ21124@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1394199526-6400-1-git-send-email-alexander.shishkin@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1394199526-6400-1-git-send-email-alexander.shishkin@linux.intel.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14031319-8236-0000-0000-000000BFF472 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Thanx, Paul > Signed-off-by: Alexander Shishkin > Cc: Paul McKenney > --- > kernel/events/core.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/kernel/events/core.c b/kernel/events/core.c > index 661951a..bce41e0 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -3861,7 +3861,7 @@ static void ring_buffer_attach(struct perf_event *event, > > 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 +3873,11 @@ 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); > + synchronize_rcu(); > + INIT_LIST_HEAD(&event->rb_entry); > } > > static void ring_buffer_wakeup(struct perf_event *event) > -- > 1.9.0 >