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 Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D755AC87FCB for ; Fri, 1 Aug 2025 19:22:06 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 1732510E30F; Fri, 1 Aug 2025 19:22:06 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.b="YzKhWppe"; dkim-atps=neutral Received: from mail-pj1-f73.google.com (mail-pj1-f73.google.com [209.85.216.73]) by gabe.freedesktop.org (Postfix) with ESMTPS id DFEBC10E30F for ; Fri, 1 Aug 2025 19:22:04 +0000 (UTC) Received: by mail-pj1-f73.google.com with SMTP id 98e67ed59e1d1-3139c0001b5so2159968a91.2 for ; Fri, 01 Aug 2025 12:22:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1754076124; x=1754680924; darn=lists.freedesktop.org; h=cc:to:from:subject:message-id:mime-version:date:from:to:cc:subject :date:message-id:reply-to; bh=mNS2Q2IorOCVtYQWCNZdPG9Q8DZamyAzOMDjYDbYBsA=; b=YzKhWppe9PACBjlXPtPmt7eoclCtWUPxA+4yNm+4x/IE259ZD7UIljmYJ1RCF5NfZJ Q/ux9TfzIk2zbj9WYQSxdiGvhCB2pKp8mV/OUqCS/PQqVPDP9n0BU6UNZ9G5oJj02+OV T0uB2RS8hDud0y8uVpLY/mDVttJIzoNEvSA3r7sE0fmiQPOQsPc4MxUeF3tVJh3Ywj4j XLiA6W+k/HH2P9vUCBebGq/RU1PlVbBx+SFa+fequ8kOFun55t8dfRX30otpjDTFwDgY EPo8blaq3cSV022lZiwdwGXF1+WjFCcQxvCAK92tS/oQ4x9seHZEtv1X1RHSfddFgk3H ejrA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1754076124; x=1754680924; h=cc:to:from:subject:message-id:mime-version:date:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=mNS2Q2IorOCVtYQWCNZdPG9Q8DZamyAzOMDjYDbYBsA=; b=ONoRBec62SvQD/k1FWpgmLJIA/xhianL88iflyfSnAIEcsv6t4FHZAp2PyYQnF8LX2 ZOyCk53/hiOU9Q2Z57Tt2ALuQEDRv62MdJiA1g+sNSIhRWwrZ0pZu4yD+ToITBEtTnL7 SX7eznkyBy2W51tcOj8pHk8jNkaHEsgEbSnqnWackq4c1fM7DxYDe5KKjkQDrM7tTGmS YnDZ0VtCKpLiLewStNQx4+tWK5JB384gru1FHyyTWjfy9AlUX6GDzmMaj/B4NIROE+6p E4K42X45rJQLixFuBCNd8ow5FICb5bufi3bvKEzqXl218mQ6V1MAKJOEiXgRFTDi2NZf +lNQ== X-Forwarded-Encrypted: i=1; AJvYcCXn0UzwmCZU5zi5G2gVsacwucwLixTalpLed2TdCOMdfxPsrv53BBA50UTvYI4tsfY1+lDo+li8+Tw=@lists.freedesktop.org X-Gm-Message-State: AOJu0YyHHgAJjm5f5G7b/LXrW2DB162jMUNzDhzdjEMFWuVEO/lwsVXd fYxT5eTUOLd5bVL9fAEntIOHTJxShUm7nvFEME+/ZVErW9ZoD6IkPlr+KIArHMcUnaGZKexQUob FIKiaxMFq X-Google-Smtp-Source: AGHT+IHAqBf/4P00GxPH9TGd4ks7immyJraYaPKAtSJjTsefHZE/ctegZQvAOdE2EzTh4JqYdZOFdCJDwMQZ X-Received: from pjbeu13.prod.google.com ([2002:a17:90a:f94d:b0:312:e914:4548]) (user=jstultz job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:3b52:b0:311:9c9a:58d7 with SMTP id 98e67ed59e1d1-321162b1060mr1309032a91.19.1754076124145; Fri, 01 Aug 2025 12:22:04 -0700 (PDT) Date: Fri, 1 Aug 2025 19:21:20 +0000 Mime-Version: 1.0 X-Mailer: git-send-email 2.50.1.565.gc32cd1483b-goog Message-ID: <20250801192157.912805-1-jstultz@google.com> Subject: [PATCH v2] locking: Fix __clear_task_blocked_on() warning from __ww_mutex_wound() path From: John Stultz To: LKML Cc: John Stultz , syzbot+602c4720aed62576cd79@syzkaller.appspotmail.com, Maarten Lankhorst , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Valentin Schneider , K Prateek Nayak , Suleiman Souhlal , airlied@gmail.com, mripard@kernel.org, simona@ffwll.ch, tzimmermann@suse.de, dri-devel@lists.freedesktop.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" The __clear_task_blocked_on() helper added a number of sanity checks ensuring we hold the mutex wait lock and that the task we are clearing blocked_on pointer (if set) matches the mutex. However, there is an edge case in the _ww_mutex_wound() logic where we need to clear the blocked_on pointer for the task that owns the mutex, not the task that is waiting on the mutex. For this case the sanity checks aren't valid, so handle this by allowing a NULL lock to skip the additional checks. This was easier to miss, I realized, as the test-ww_mutex driver only exercises the wait-die class of ww_mutexes. I've got a follow up patch to extend the test so that it will exercise both. Fixes: a4f0b6fef4b0 ("locking/mutex: Add p->blocked_on wrappers for correctness checks") Reported-by: syzbot+602c4720aed62576cd79@syzkaller.appspotmail.com Reported-by: Maarten Lankhorst Closes: https://lore.kernel.org/lkml/68894443.a00a0220.26d0e1.0015.GAE@google.com/ Signed-off-by: John Stultz --- v2: * Rewording of "lock" to "mutex" in commit and comment for clarity * Rework __clear_task_blocked_on() to use READ_ONCE and WRITE_ONCE so we don't trip over the WARNING if two instances race, as suggested by K Prateek Nayak and Maarten Lankhorst Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Juri Lelli Cc: Vincent Guittot Cc: Dietmar Eggemann Cc: Valentin Schneider Cc: K Prateek Nayak Cc: Suleiman Souhlal Cc: Maarten Lankhorst Cc: airlied@gmail.com Cc: mripard@kernel.org Cc: simona@ffwll.ch Cc: tzimmermann@suse.de Cc: dri-devel@lists.freedesktop.org Cc: kernel-team@android.com --- include/linux/sched.h | 23 +++++++++++++---------- kernel/locking/ww_mutex.h | 6 +++++- 2 files changed, 18 insertions(+), 11 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index 40d2fa90df425..700b50d29f7fe 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -2166,16 +2166,19 @@ static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m) static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m) { - WARN_ON_ONCE(!m); - /* Currently we serialize blocked_on under the mutex::wait_lock */ - lockdep_assert_held_once(&m->wait_lock); - /* - * There may be cases where we re-clear already cleared - * blocked_on relationships, but make sure we are not - * clearing the relationship with a different lock. - */ - WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m); - p->blocked_on = NULL; + if (m) { + struct mutex *blocked_on = READ_ONCE(p->blocked_on); + + /* Currently we serialize blocked_on under the mutex::wait_lock */ + lockdep_assert_held_once(&m->wait_lock); + /* + * There may be cases where we re-clear already cleared + * blocked_on relationships, but make sure we are not + * clearing the relationship with a different lock. + */ + WARN_ON_ONCE(blocked_on && blocked_on != m); + } + WRITE_ONCE(p->blocked_on, NULL); } static inline void clear_task_blocked_on(struct task_struct *p, struct mutex *m) diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h index 086fd5487ca77..31a785afee6c0 100644 --- a/kernel/locking/ww_mutex.h +++ b/kernel/locking/ww_mutex.h @@ -342,8 +342,12 @@ static bool __ww_mutex_wound(struct MUTEX *lock, * When waking up the task to wound, be sure to clear the * blocked_on pointer. Otherwise we can see circular * blocked_on relationships that can't resolve. + * + * NOTE: We pass NULL here instead of lock, because we + * are waking the mutex owner, who may be currently + * blocked on a different mutex. */ - __clear_task_blocked_on(owner, lock); + __clear_task_blocked_on(owner, NULL); wake_q_add(wake_q, owner); } return true; -- 2.50.1.565.gc32cd1483b-goog