From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 19405C433E9 for ; Wed, 17 Mar 2021 03:02:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E193C64E33 for ; Wed, 17 Mar 2021 03:02:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229871AbhCQDCT (ORCPT ); Tue, 16 Mar 2021 23:02:19 -0400 Received: from mx2.suse.de ([195.135.220.15]:57056 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229803AbhCQDBv (ORCPT ); Tue, 16 Mar 2021 23:01:51 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A0CB2AC17; Wed, 17 Mar 2021 03:01:49 +0000 (UTC) Date: Tue, 16 Mar 2021 20:01:41 -0700 From: Davidlohr Bueso To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , "Paul E. McKenney" , linux-kernel@vger.kernel.org, Juri Lelli Subject: Re: [PATCH 3/4] locking/ww_mutex: Treat ww_mutex_lock() like a trylock Message-ID: <20210317030141.hsfeodb7toihrvrq@offworld> References: <20210316153119.13802-1-longman@redhat.com> <20210316153119.13802-4-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20210316153119.13802-4-longman@redhat.com> User-Agent: NeoMutt/20201120 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 16 Mar 2021, Waiman Long wrote: >It was found that running the ww_mutex_lock-torture test produced the >following lockdep splat almost immediately: > >[ 103.892638] ====================================================== >[ 103.892639] WARNING: possible circular locking dependency detected >[ 103.892641] 5.12.0-rc3-debug+ #2 Tainted: G S W >[ 103.892643] ------------------------------------------------------ >[ 103.892643] lock_torture_wr/3234 is trying to acquire lock: >[ 103.892646] ffffffffc0b35b10 (torture_ww_mutex_2.base){+.+.}-{3:3}, at: torture_ww_mutex_lock+0x316/0x720 [locktorture] >[ 103.892660] >[ 103.892660] but task is already holding lock: >[ 103.892661] ffffffffc0b35cd0 (torture_ww_mutex_0.base){+.+.}-{3:3}, at: torture_ww_mutex_lock+0x3e2/0x720 [locktorture] >[ 103.892669] >[ 103.892669] which lock already depends on the new lock. >[ 103.892669] >[ 103.892670] >[ 103.892670] the existing dependency chain (in reverse order) is: >[ 103.892671] >[ 103.892671] -> #2 (torture_ww_mutex_0.base){+.+.}-{3:3}: >[ 103.892675] lock_acquire+0x1c5/0x830 >[ 103.892682] __ww_mutex_lock.constprop.15+0x1d1/0x2e50 >[ 103.892687] ww_mutex_lock+0x4b/0x180 >[ 103.892690] torture_ww_mutex_lock+0x316/0x720 [locktorture] >[ 103.892694] lock_torture_writer+0x142/0x3a0 [locktorture] >[ 103.892698] kthread+0x35f/0x430 >[ 103.892701] ret_from_fork+0x1f/0x30 >[ 103.892706] >[ 103.892706] -> #1 (torture_ww_mutex_1.base){+.+.}-{3:3}: >[ 103.892709] lock_acquire+0x1c5/0x830 >[ 103.892712] __ww_mutex_lock.constprop.15+0x1d1/0x2e50 >[ 103.892715] ww_mutex_lock+0x4b/0x180 >[ 103.892717] torture_ww_mutex_lock+0x316/0x720 [locktorture] >[ 103.892721] lock_torture_writer+0x142/0x3a0 [locktorture] >[ 103.892725] kthread+0x35f/0x430 >[ 103.892727] ret_from_fork+0x1f/0x30 >[ 103.892730] >[ 103.892730] -> #0 (torture_ww_mutex_2.base){+.+.}-{3:3}: >[ 103.892733] check_prevs_add+0x3fd/0x2470 >[ 103.892736] __lock_acquire+0x2602/0x3100 >[ 103.892738] lock_acquire+0x1c5/0x830 >[ 103.892740] __ww_mutex_lock.constprop.15+0x1d1/0x2e50 >[ 103.892743] ww_mutex_lock+0x4b/0x180 >[ 103.892746] torture_ww_mutex_lock+0x316/0x720 [locktorture] >[ 103.892749] lock_torture_writer+0x142/0x3a0 [locktorture] >[ 103.892753] kthread+0x35f/0x430 >[ 103.892755] ret_from_fork+0x1f/0x30 >[ 103.892757] >[ 103.892757] other info that might help us debug this: >[ 103.892757] >[ 103.892758] Chain exists of: >[ 103.892758] torture_ww_mutex_2.base --> torture_ww_mutex_1.base --> torture_ww_mutex_0.base >[ 103.892758] >[ 103.892763] Possible unsafe locking scenario: >[ 103.892763] >[ 103.892764] CPU0 CPU1 >[ 103.892765] ---- ---- >[ 103.892765] lock(torture_ww_mutex_0.base); >[ 103.892767] lock(torture_ww_mutex_1.base); >[ 103.892770] lock(torture_ww_mutex_0.base); >[ 103.892772] lock(torture_ww_mutex_2.base); >[ 103.892774] >[ 103.892774] *** DEADLOCK *** > >Since ww_mutex is supposed to be deadlock-proof if used properly, such >deadlock scenario should not happen. To avoid this false positive splat, >treat ww_mutex_lock() like a trylock(). > >After applying this patch, the locktorture test can run for a long time >without triggering the circular locking dependency splat. > >Signed-off-by: Waiman Long Acked-by Davidlohr Bueso