Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752973AbbLKHdN (ORCPT ); Fri, 11 Dec 2015 02:33:13 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:42613 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751965AbbLKHdM (ORCPT ); Fri, 11 Dec 2015 02:33:12 -0500 Date: Fri, 11 Dec 2015 08:33:01 +0100 From: Peter Zijlstra To: Andrew Pinski Cc: Will Deacon , Davidlohr Bueso , Thomas Gleixner , "Paul E. McKenney" , Ingo Molnar , Linux Kernel Mailing List , "linux-arm-kernel@lists.infradead.org" , Andrew Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX) Message-ID: <20151211073301.GD6356@twins.programming.kicks-ass.net> References: <5669D5F2.5050004@caviumnetworks.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Content-Length: 3925 Lines: 94 On Thu, Dec 10, 2015 at 07:29:34PM -0800, Andrew Pinski wrote: > On Thu, Dec 10, 2015 at 11:44 AM, David Danny wrote: > > > > Hi, > > > > We are getting soft lockup OOPs on Cavium CN88XX (A.K.A. ThunderX), which is an arm64 implementation. > > I get a slightly different OOPs and reverting > c55a6ffa6285e29f874ed403979472631ec70bff I was able to boot. > What I saw with osq_lock.c was that osq_wait_next is called for both > lock and unlock case so it might need both barriers. > The other question comes does atomic_cmpxchg_release have release > semantics when the compare fails? Right now it does not. > Out cmpxchg primites imply no barrier on failure, this is documented somewhere.. /me searches.. --- commit ed2de9f74ecbbf3063d29b2334e7b455d7f35189 Author: Will Deacon Date: Thu Jul 16 16:10:06 2015 +0100 locking/Documentation: Clarify failed cmpxchg() memory ordering semantics A failed cmpxchg does not provide any memory ordering guarantees, a property that is used to optimise the cmpxchg implementations on Alpha, PowerPC and arm64. This patch updates atomic_ops.txt and memory-barriers.txt to reflect this. Signed-off-by: Will Deacon Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Davidlohr Bueso Cc: Douglas Hatch Cc: H. Peter Anvin Cc: Jonathan Corbet Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Scott J Norton Cc: Thomas Gleixner Cc: Waiman Long Link: http://lkml.kernel.org/r/20150716151006.GH26390@arm.com Signed-off-by: Ingo Molnar diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt index dab6da3382d9..b19fc34efdb1 100644 --- a/Documentation/atomic_ops.txt +++ b/Documentation/atomic_ops.txt @@ -266,7 +266,9 @@ with the given old and new values. Like all atomic_xxx operations, atomic_cmpxchg will only satisfy its atomicity semantics as long as all other accesses of *v are performed through atomic_xxx operations. -atomic_cmpxchg must provide explicit memory barriers around the operation. +atomic_cmpxchg must provide explicit memory barriers around the operation, +although if the comparison fails then no memory ordering guarantees are +required. The semantics for atomic_cmpxchg are the same as those defined for 'cas' below. diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 13feb697271f..18fc860df1be 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -2383,9 +2383,7 @@ about the state (old or new) implies an SMP-conditional general memory barrier explicit lock operations, described later). These include: xchg(); - cmpxchg(); atomic_xchg(); atomic_long_xchg(); - atomic_cmpxchg(); atomic_long_cmpxchg(); atomic_inc_return(); atomic_long_inc_return(); atomic_dec_return(); atomic_long_dec_return(); atomic_add_return(); atomic_long_add_return(); @@ -2398,7 +2396,9 @@ about the state (old or new) implies an SMP-conditional general memory barrier test_and_clear_bit(); test_and_change_bit(); - /* when succeeds (returns 1) */ + /* when succeeds */ + cmpxchg(); + atomic_cmpxchg(); atomic_long_cmpxchg(); atomic_add_unless(); atomic_long_add_unless(); These are used for such things as implementing ACQUIRE-class and RELEASE-class -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/