From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752696AbdF3UEz (ORCPT ); Fri, 30 Jun 2017 16:04:55 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:55330 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751982AbdF3UEy (ORCPT ); Fri, 30 Jun 2017 16:04:54 -0400 Date: Fri, 30 Jun 2017 13:04:48 -0700 From: "Paul E. McKenney" To: Alan Stern Cc: Oleg Nesterov , linux-kernel@vger.kernel.org, netfilter-devel@vger.kernel.org, netdev@vger.kernel.org, akpm@linux-foundation.org, mingo@redhat.com, dave@stgolabs.net, manfred@colorfullife.com, tj@kernel.org, arnd@arndb.de, linux-arch@vger.kernel.org, will.deacon@arm.com, peterz@infradead.org, parri.andrea@gmail.com, torvalds@linux-foundation.org Subject: Re: [PATCH RFC 02/26] task_work: Replace spin_unlock_wait() with lock/unlock pair Reply-To: paulmck@linux.vnet.ibm.com References: <20170630192123.GA8471@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17063020-0052-0000-0000-00000233E1C9 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007299; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000214; SDB=6.00881006; UDB=6.00439251; IPR=6.00661159; BA=6.00005448; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016025; XFM=3.00000015; UTC=2017-06-30 20:04:51 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17063020-0053-0000-0000-00005129F8FA Message-Id: <20170630200448.GG2393@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-06-30_13:,, 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-1706300314 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 30, 2017 at 03:50:33PM -0400, Alan Stern wrote: > On Fri, 30 Jun 2017, Oleg Nesterov wrote: > > > On 06/30, Paul E. McKenney wrote: > > > > > > On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote: > > > > > > > > I do not think the overhead will be noticeable in this particular case. > > > > > > > > But I am not sure I understand why do we want to unlock_wait. Yes I agree, > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > > > if it was not clear, I tried to say "why do we want to _remove_ unlock_wait". > > > > > > it has some problems, but still... > > > > > > > > The code above looks strange for me. If we are going to repeat this pattern > > > > the perhaps we should add a helper for lock+unlock and name it unlock_wait2 ;) > > > > > > > > If not, we should probably change this code more: > > > > > > This looks -much- better than my patch! May I have your Signed-off-by? > > > > Only if you promise to replace all RCU flavors with a single simple implementation > > based on rwlock ;) > > > > Seriously, of course I won't argue, and it seems that nobody except me likes > > this primitive, but to me spin_unlock_wait() looks like synchronize_rcu(() and > > sometimes it makes sense. > > If it looks like synchronize_rcu(), why not actually use > synchronize_rcu()? My guess is that the latencies of synchronize_rcu() don't suit his needs. When the lock is not held, spin_unlock_wait() is quite fast, even compared to expedited grace periods. Thanx, Paul > Alan Stern > > > Including this particular case. task_work_run() is going to flush/destroy the > > ->task_works list, so it needs to wait until all currently executing "readers" > > (task_work_cancel()'s which have started before ->task_works was updated) have > > completed. >