From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752671AbdLNOMo (ORCPT ); Thu, 14 Dec 2017 09:12:44 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:51103 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751029AbdLNOMn (ORCPT ); Thu, 14 Dec 2017 09:12:43 -0500 Date: Thu, 14 Dec 2017 06:12:40 -0800 From: Matthew Wilcox To: Christoph Hellwig Cc: Andrew Morton , kernel test robot , LKP , linux-kernel@vger.kernel.org, Linux Memory Management List , wfg@linux.intel.com, Stephen Rothwell Subject: Re: d1fc031747 ("sched/wait: assert the wait_queue_head lock is .."): EIP: __wake_up_common Message-ID: <20171214141240.GD30288@bombadil.infradead.org> References: <5a31cac7.i9WLKx5al8+rBn73%fengguang.wu@intel.com> <20171213170300.b0bb26900dd00641819b4872@linux-foundation.org> <20171214125809.GB30288@bombadil.infradead.org> <20171214131037.GD10791@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171214131037.GD10791@lst.de> User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 14, 2017 at 02:10:37PM +0100, Christoph Hellwig wrote: > On Thu, Dec 14, 2017 at 04:58:09AM -0800, Matthew Wilcox wrote: > > Looks pretty clear to me that userfaultfd is also abusing the wake_up_locked > > interfaces: > > > > spin_lock(&ctx->fault_pending_wqh.lock); > > __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range); > > __wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range); > > spin_unlock(&ctx->fault_pending_wqh.lock); > > > > Sure, it's locked, but not by the lock you thought it was going to be. > > > > There doesn't actually appear to be a bug here; fault_wqh is always serialised > > by fault_pending_wqh.lock, but lockdep can't know that. I think this patch > > will solve the problem. > > Or userfaultfd could just always use the waitqueue lock, similar to what > we are doing in epoll. > > But unless someone care about micro-optimizatations I'm tempted to > add your patch to the next iteration of the series. userfaultfd is using the waitqueue lock -- it just has two waitqueues that it's protecting with the same lock. If the patch goes through as-is, try this changelog: [PATCH] userfaultfd: Use fault_wqh lock userfaultfd was using the fault_pending_wq lock to protect both fault_pending_wq and fault_wqh. With Christoph's addition of a lockdep assert to the wait queue code, that will trigger warnings (although there is no bug). Remove the warning by using __wake_up which will take the fault_wqh lock. This lock now nests inside the fault_pending_wqh lock, but that's not a problem since it was entireyl unused before. Signed-off-by: Matthew Wilcox