From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: [RFC PATCH] fs: Use a seperate wq for do_sync_work() to avoid a potential deadlock Date: Wed, 17 Sep 2014 20:22:02 +0200 Message-ID: <20140917182202.GE19308@redhat.com> References: <1410953942-32144-1-git-send-email-atomlin@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, david@fromorbit.com, bmr@redhat.com, jcastillo@redhat.com, mguzik@redhat.com, linux-kernel@vger.kernel.org To: Aaron Tomlin Return-path: Content-Disposition: inline In-Reply-To: <1410953942-32144-1-git-send-email-atomlin@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On 09/17, Aaron Tomlin wrote: > > Since do_sync_work() is a deferred function it can block indefinitely by > design. At present do_sync_work() is added to the global system_wq. > As such a deadlock is theoretically possible between sys_unmount() and > sync_filesystems(): > > * The current work fn on the system_wq (do_sync_work()) is blocked > waiting to aquire a sb's s_umount for reading. > > * The "umount" task is the current owner of the s_umount in > question but is waiting for do_sync_work() to continue. > Thus we hit a deadlock situation. > I can't comment the patches in this area, but I am just curious... Could you explain this deadlock in more details? I simply can't understand what "waiting for do_sync_work()" actually means. > This patch introduces a separate workqueue for do_sync_work() to avoid a > the described deadlock. The subject and the changelog do not match the patch, it doesn't add/use another workqueue. Oleg.