From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759796AbbA2KYM (ORCPT ); Thu, 29 Jan 2015 05:24:12 -0500 Received: from ip4-83-240-67-251.cust.nbox.cz ([83.240.67.251]:44737 "EHLO ip4-83-240-18-248.cust.nbox.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1756495AbbA2JAn (ORCPT ); Thu, 29 Jan 2015 04:00:43 -0500 From: Jiri Slaby To: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Chris Wilson , Jani Nikula , Jiri Slaby Subject: [PATCH 3.12 145/176] drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES Date: Wed, 28 Jan 2015 15:29:35 +0100 Message-Id: <159544d674449fef4f9e862501ae4d6f80f2adce.1422455352.git.jslaby@suse.cz> X-Mailer: git-send-email 2.2.2 In-Reply-To: References: In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Chris Wilson 3.12-stable review patch. If anyone has any objections, please let me know. =============== commit 226e5ae9e5f9108beb0bde4ac69f68fe6210fed9 upstream. If CONFIG_DEBUG_MUTEXES is set, the mutex->owner field is only cleared if the mutex debugging is enabled which introduces a race in our mutex_is_locked_by() - i.e. we may inspect the old owner value before it is acquired by the new task. This is the root cause of this error: # diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c # index 5cf6731..3ef3736 100644 # --- a/kernel/locking/mutex-debug.c # +++ b/kernel/locking/mutex-debug.c # @@ -80,13 +80,13 @@ void debug_mutex_unlock(struct mutex *lock) # DEBUG_LOCKS_WARN_ON(lock->owner != current); # # DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next); # - mutex_clear_owner(lock); # } # # /* # * __mutex_slowpath_needs_to_unlock() is explicitly 0 for debug # * mutexes so that we can do it here after we've verified state. # */ # + mutex_clear_owner(lock); # atomic_set(&lock->count, 1); # } Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87955 Signed-off-by: Chris Wilson Reviewed-by: Daniel Vetter Signed-off-by: Jani Nikula Signed-off-by: Jiri Slaby --- drivers/gpu/drm/i915/i915_gem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index ef5fe7e9d86c..7e4e6896fa81 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -4795,7 +4795,7 @@ static bool mutex_is_locked_by(struct mutex *mutex, struct task_struct *task) if (!mutex_is_locked(mutex)) return false; -#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_MUTEXES) +#if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES) return mutex->owner == task; #else /* Since UP may be pre-empted, we cannot assume that we own the lock */ -- 2.2.2