From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756774AbbFPLQh (ORCPT ); Tue, 16 Jun 2015 07:16:37 -0400 Received: from casper.infradead.org ([85.118.1.10]:48752 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752270AbbFPLQ3 (ORCPT ); Tue, 16 Jun 2015 07:16:29 -0400 Date: Tue, 16 Jun 2015 13:16:20 +0200 From: Peter Zijlstra To: Oleg Nesterov 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: <20150616111620.GC18673@twins.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150616110855.GM3644@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I made that the below. Are you OK with the Changelog edits and the added SoB ? --- 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);