From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752976AbdGCWaV (ORCPT ); Mon, 3 Jul 2017 18:30:21 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:38275 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752237AbdGCWaR (ORCPT ); Mon, 3 Jul 2017 18:30:17 -0400 Date: Mon, 3 Jul 2017 15:30:11 -0700 From: "Paul E. McKenney" To: Will Deacon Cc: Linus Torvalds , Linux Kernel Mailing List , NetFilter , Network Development , Oleg Nesterov , Andrew Morton , Ingo Molnar , Davidlohr Bueso , Manfred Spraul , Tejun Heo , Arnd Bergmann , "linux-arch@vger.kernel.org" , Peter Zijlstra , Alan Stern , Andrea Parri Subject: Re: [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions Reply-To: paulmck@linux.vnet.ibm.com References: <20170629235918.GA6445@linux.vnet.ibm.com> <1498780894-8253-8-git-send-email-paulmck@linux.vnet.ibm.com> <20170630091928.GC9726@arm.com> <20170630123815.GT2393@linux.vnet.ibm.com> <20170630131339.GA14118@arm.com> <20170630221840.GI2393@linux.vnet.ibm.com> <20170703131514.GE1573@arm.com> <20170703161851.GY2393@linux.vnet.ibm.com> <20170703171338.GG1573@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170703171338.GG1573@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17070322-0008-0000-0000-00000254E3ED X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007315; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000214; SDB=6.00882421; UDB=6.00440109; IPR=6.00662612; BA=6.00005451; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016062; XFM=3.00000015; UTC=2017-07-03 22:30:15 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17070322-0009-0000-0000-000035E63CC3 Message-Id: <20170703223011.GI2393@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-07-03_14:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1707030367 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 03, 2017 at 06:13:38PM +0100, Will Deacon wrote: > On Mon, Jul 03, 2017 at 09:40:22AM -0700, Linus Torvalds wrote: > > On Mon, Jul 3, 2017 at 9:18 AM, Paul E. McKenney > > wrote: > > > > > > Agreed, and my next step is to look at spin_lock() followed by > > > spin_is_locked(), not necessarily the same lock. > > > > Hmm. Most (all?) "spin_is_locked()" really should be about the same > > thread that took the lock (ie it's about asserts and lock debugging). > > > > The optimistic ABBA avoidance pattern for spinlocks *should* be > > > > spin_lock(inner) > > ... > > if (!try_lock(outer)) { > > spin_unlock(inner); > > .. do them in the right order .. > > > > so I don't think spin_is_locked() should have any memory barriers. > > > > In fact, the core function for spin_is_locked() is arguably > > arch_spin_value_unlocked() which doesn't even do the access itself. > > Yeah, but there's some spaced-out stuff going on in kgdb_cpu_enter where > it looks to me like raw_spin_is_locked is used for synchronization. My > eyes are hurting looking at it, though. That certainly is one interesting function, isn't it? I wonder what happens if you replace the raw_spin_is_locked() calls with an unlock under a trylock check? ;-) Thanx, Paul