From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756215AbbFPTFF (ORCPT ); Tue, 16 Jun 2015 15:05:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48654 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752809AbbFPTFA (ORCPT ); Tue, 16 Jun 2015 15:05:00 -0400 Date: Tue, 16 Jun 2015 21:03:51 +0200 From: Oleg Nesterov To: Peter Zijlstra Cc: "Paul E. McKenney" , tj@kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org, der.herr@hofr.at, dave@stgolabs.net, torvalds@linux-foundation.org, josh@joshtriplett.org Subject: Re: ring_buffer_attach && cond_synchronize_rcu (Was: percpu-rwsem: Optimize readers and reduce global impact) Message-ID: <20150616190351.GA28724@redhat.com> References: <20150526114356.609107918@infradead.org> <20150526120215.042527659@infradead.org> <20150530171806.GB14999@linux.vnet.ibm.com> <20150530200425.GA15748@redhat.com> <20150616110855.GM3644@twins.programming.kicks-ass.net> <20150616111620.GC18673@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150616111620.GC18673@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/16, Peter Zijlstra wrote: > > > I made that the below. > > Are you OK with the Changelog edits and the added SoB ? Of course, Thanks Peter! Oleg. > --- > Subject: perf: Fix ring_buffer_attach() RCU sync, again. > From: Oleg Nesterov > Date: Sat, 30 May 2015 22:04:25 +0200 > > While looking for other users of get_state/cond_sync. I Found > ring_buffer_attach() and it looks obviously buggy? > > 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? > > It also moves the ->rcu_pending check under "if (rb)", to make it > more readable imo. > > Cc: tj@kernel.org > Cc: mingo@redhat.com > Cc: der.herr@hofr.at > Cc: dave@stgolabs.net > Cc: torvalds@linux-foundation.org > Cc: josh@joshtriplett.org > Cc: "Paul E. McKenney" > Cc: Alexander Shishkin > Fixes: b69cf53640da ("perf: Fix a race between ring_buffer_detach() and ring_buffer_attach()") > Signed-off-by: Oleg Nesterov > Signed-off-by: Peter Zijlstra (Intel) > Link: http://lkml.kernel.org/r/20150530200425.GA15748@redhat.com > --- > kernel/events/core.c | 14 +++++++------- > 1 file changed, 7 insertions(+), 7 deletions(-) > > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -4307,20 +4307,20 @@ 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);