From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754446AbbJNUWP (ORCPT ); Wed, 14 Oct 2015 16:22:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33972 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753579AbbJNUWM (ORCPT ); Wed, 14 Oct 2015 16:22:12 -0400 Subject: Re: [PATCH 0/3] SCSI: Fix hard lockup in scsi_remove_target() From: Ewan Milne Reply-To: emilne@redhat.com To: James Bottomley Cc: Johannes Thumshirn , Christoph Hellwig , Hannes Reinecke , linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <1444833036.2220.38.camel@HansenPartnership.com> References: <1444833036.2220.38.camel@HansenPartnership.com> Content-Type: text/plain; charset="UTF-8" Organization: Red Hat Date: Wed, 14 Oct 2015 16:22:10 -0400 Message-ID: <1444854130.26884.33.camel@localhost.localdomain> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2015-10-14 at 07:30 -0700, James Bottomley wrote: > On Wed, 2015-10-14 at 15:50 +0200, Johannes Thumshirn wrote: > > Removing a SCSI target via scsi_remove_target() suspected to be racy. When a > > sibling get's removed from the list it can occassionly happen that one CPU is > > stuck endlessly looping around this code block > > > > list_for_each_entry(starget, &shost->__targets, siblings) { > > if (starget->state == STARGET_DEL) > > continue; > > How long is the __targets list? It seems a bit unlikely that this is > the exact cause, because for a short list all in STARGET_DEL that loop > should exit very quickly. Where in the code does scsi_remove_target > +0x68/0x240 actually point to? > > Is it not a bit more likely that we're following a removed list element? > Since that points back to itself, the list_for_each_entry() would then > circulate forever. If that's the case the simple fix would be to use > the safe version of the list traversal macro. > > James For what it's worth, I've seen a dump where this was exactly the case. starget was in STARGET_DEL state, starget->siblings pointed to itself, kref was 0, reap_ref was 0 (this was a while back). The problem was not able to be reproduced at the time. -Ewan