From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752358AbdLLTgJ (ORCPT ); Tue, 12 Dec 2017 14:36:09 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:56556 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752209AbdLLTgI (ORCPT ); Tue, 12 Dec 2017 14:36:08 -0500 Date: Tue, 12 Dec 2017 11:36:02 -0800 From: "Paul E. McKenney" To: Russell King - ARM Linux Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Peng Fan , Fabio Estevam , Ingo Molnar , "Peter Zijlstra (Intel)" , Michal Hocko , Thomas Gleixner Subject: Re: [PATCH] ARM: CPU hotplug: Delegate complete() to surviving CPU Reply-To: paulmck@linux.vnet.ibm.com References: <20171212172059.GA11875@linux.vnet.ibm.com> <20171212173759.GE10595@n2100.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171212173759.GE10595@n2100.armlinux.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17121219-0024-0000-0000-0000030054C7 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008194; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000244; SDB=6.00959264; UDB=6.00485119; IPR=6.00739286; BA=6.00005738; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018506; XFM=3.00000015; UTC=2017-12-12 19:36:05 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17121219-0025-0000-0000-0000464BE397 Message-Id: <20171212193602.GG7829@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-12-12_10:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1712120278 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 12, 2017 at 05:37:59PM +0000, Russell King - ARM Linux wrote: > On Tue, Dec 12, 2017 at 09:20:59AM -0800, Paul E. McKenney wrote: > > The ARM implementation of arch_cpu_idle_dead() invokes complete(), but > > does so after RCU has stopped watching the outgoing CPU, which results > > in lockdep complaints because complete() invokes functions containing RCU > > readers. This patch therefore uses Thomas Gleixner's trick of delegating > > the complete() call to a surviving CPU via smp_call_function_single(). > > > > Reported-by: Peng Fan > > Reported-by: Russell King - ARM Linux > > Signed-off-by: Paul E. McKenney > > Tested-by: Tested-by: Fabio Estevam > > Cc: Russell King > > Cc: Ingo Molnar > > Cc: "Peter Zijlstra (Intel)" > > Cc: Michal Hocko > > Cc: Thomas Gleixner > > Cc: > > As I just described in response to Fabio's testing, this doesn't solve > anything if CONFIG_BL_SWITCHER is enabled. We could lose the unlock of > a spinlock in the GIC code for sending the IPI. As I already said > previously in our discussion (but I guess you just don't believe me): Sorry, Russell, but most days I don't even believe myself. So it is nothing personal, just one of the occupational hazards of being me. > "2. there's some optional locking in the GIC driver that cause problems > for the cpu dying path. > > The concensus last time around was that the IPI solution is a non- > starter, so the seven year proven-reliable solution (disregarding the > RCU warning) persists because I don't think anyone came up with a > better solution." > > Using smp_call_function_single() invokes the IPI paths. OK, another approach is to have the dying CPU simply set an in-memory flag, which a surviving CPU polls for. There are of course any number of ways of doing the polling loop. So what bad thing happens when you use that approach? Thanx, Paul