From mboxrd@z Thu Jan 1 00:00:00 1970 From: peterz@infradead.org (Peter Zijlstra) Date: Thu, 5 Apr 2018 19:07:06 +0200 Subject: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath In-Reply-To: <1522947547-24081-3-git-send-email-will.deacon@arm.com> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-3-git-send-email-will.deacon@arm.com> Message-ID: <20180405170706.GC4082@hirez.programming.kicks-ass.net> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Apr 05, 2018 at 05:58:59PM +0100, Will Deacon wrote: > The qspinlock locking slowpath utilises a "pending" bit as a simple form > of an embedded test-and-set lock that can avoid the overhead of explicit > queuing in cases where the lock is held but uncontended. This bit is > managed using a cmpxchg loop which tries to transition the uncontended > lock word from (0,0,0) -> (0,0,1) or (0,0,1) -> (0,1,1). > > Unfortunately, the cmpxchg loop is unbounded and lockers can be starved > indefinitely if the lock word is seen to oscillate between unlocked > (0,0,0) and locked (0,0,1). This could happen if concurrent lockers are > able to take the lock in the cmpxchg loop without queuing and pass it > around amongst themselves. > > This patch fixes the problem by unconditionally setting _Q_PENDING_VAL > using atomic_fetch_or, Of course, LL/SC or cmpxchg implementations of fetch_or do not in fact get anything from this ;-) From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751487AbeDERHR (ORCPT ); Thu, 5 Apr 2018 13:07:17 -0400 Received: from merlin.infradead.org ([205.233.59.134]:58982 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750786AbeDERHQ (ORCPT ); Thu, 5 Apr 2018 13:07:16 -0400 Date: Thu, 5 Apr 2018 19:07:06 +0200 From: Peter Zijlstra To: Will Deacon Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@kernel.org, boqun.feng@gmail.com, paulmck@linux.vnet.ibm.com, catalin.marinas@arm.com, Waiman Long Subject: Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Message-ID: <20180405170706.GC4082@hirez.programming.kicks-ass.net> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-3-git-send-email-will.deacon@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1522947547-24081-3-git-send-email-will.deacon@arm.com> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 05, 2018 at 05:58:59PM +0100, Will Deacon wrote: > The qspinlock locking slowpath utilises a "pending" bit as a simple form > of an embedded test-and-set lock that can avoid the overhead of explicit > queuing in cases where the lock is held but uncontended. This bit is > managed using a cmpxchg loop which tries to transition the uncontended > lock word from (0,0,0) -> (0,0,1) or (0,0,1) -> (0,1,1). > > Unfortunately, the cmpxchg loop is unbounded and lockers can be starved > indefinitely if the lock word is seen to oscillate between unlocked > (0,0,0) and locked (0,0,1). This could happen if concurrent lockers are > able to take the lock in the cmpxchg loop without queuing and pass it > around amongst themselves. > > This patch fixes the problem by unconditionally setting _Q_PENDING_VAL > using atomic_fetch_or, Of course, LL/SC or cmpxchg implementations of fetch_or do not in fact get anything from this ;-)