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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EDCAAC19F2A for ; Thu, 11 Aug 2022 11:33:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235058AbiHKLdP (ORCPT ); Thu, 11 Aug 2022 07:33:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34170 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234935AbiHKLcz (ORCPT ); Thu, 11 Aug 2022 07:32:55 -0400 Received: from www262.sakura.ne.jp (www262.sakura.ne.jp [202.181.97.72]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3A3E2252BC for ; Thu, 11 Aug 2022 04:32:31 -0700 (PDT) Received: from fsav415.sakura.ne.jp (fsav415.sakura.ne.jp [133.242.250.114]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id 27BBWToO032136; Thu, 11 Aug 2022 20:32:30 +0900 (JST) (envelope-from penguin-kernel@I-love.SAKURA.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav415.sakura.ne.jp (F-Secure/fsigk_smtp/550/fsav415.sakura.ne.jp); Thu, 11 Aug 2022 20:32:29 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/550/fsav415.sakura.ne.jp) Received: from [192.168.1.9] (M106072142033.v4.enabler.ne.jp [106.72.142.33]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id 27BBWSS6032129 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NO); Thu, 11 Aug 2022 20:32:29 +0900 (JST) (envelope-from penguin-kernel@I-love.SAKURA.ne.jp) Message-ID: <82af40cc-bf85-2b53-b8f9-dfc12e66a781@I-love.SAKURA.ne.jp> Date: Thu, 11 Aug 2022 20:32:28 +0900 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.1.2 Content-Language: en-US To: Peter Zijlstra , Ingo Molnar , Will Deacon , Waiman Long , Boqun Feng Cc: Thomas Gleixner , Shaokun Zhang , Sebastian Andrzej Siewior , Petr Mladek , Andrew Morton , Ben Dooks , Rasmus Villemoes , Luis Chamberlain , Xiaoming Ni , John Ogness , LKML , syzkaller-bugs From: Tetsuo Handa Subject: [PATCH] locking/lockdep: add debug_show_all_lock_holders() Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Currently, check_hung_uninterruptible_tasks() reports details of locks held in the system. Also, lockdep_print_held_locks() does not report details of locks held by a thread if that thread is in TASK_RUNNING state. Several years of experience of debugging without vmcore tells me that these limitations have been a barrier for understanding what went wrong in syzbot's "INFO: task hung in" reports. I initially thought that the cause of "INFO: task hung in" reports is due to over-stressing. But I understood that over-stressing is unlikely. I now consider that there likely is a deadlock/livelock bug where lockdep cannot report as a deadlock when "INFO: task hung in" is reported. A typical case is that thread-1 is waiting for something to happen (e.g. wait_event_*()) with a lock held. When thread-2 tries to hold that lock using e.g. mutex_lock(), check_hung_uninterruptible_tasks() reports that thread-2 is hung and thread-1 is holding a lock which thread-2 is trying to hold. But currently check_hung_uninterruptible_tasks() cannot report the exact location of thread-1 which gives us an important hint for understanding why thread-1 is holding that lock for so long period. When check_hung_uninterruptible_tasks() reports a thread waiting for a lock, it is important to report backtrace of threads which already held that lock. Therefore, allow check_hung_uninterruptible_tasks() to report the exact location of threads which is holding any lock. Signed-off-by: Tetsuo Handa --- I wish that lockdep continues tracking locks even after something went wrong (i.e. debug_locks remains 1), for recently I sometimes encounter problems that disable lockdep during boot stage. It would be noisy to report possibility of e.g. circular locking dependency every time due to keeping debug_locks enabled. But tracking locks even after something went wrong will help debug_show_all_lock_holders() to survive problems during boot stage. Changing debug_locks behavior is a future patch. For now, this patch alone will help debugging Greg's usb.git#usb-testing tree which is generating many "INFO: task hung in" reports. include/linux/debug_locks.h | 5 +++++ kernel/hung_task.c | 2 +- kernel/locking/lockdep.c | 32 ++++++++++++++++++++++++++++++++ 3 files changed, 38 insertions(+), 1 deletion(-) diff --git a/include/linux/debug_locks.h b/include/linux/debug_locks.h index dbb409d77d4f..0567d5ce5b4a 100644 --- a/include/linux/debug_locks.h +++ b/include/linux/debug_locks.h @@ -50,6 +50,7 @@ extern int debug_locks_off(void); #ifdef CONFIG_LOCKDEP extern void debug_show_all_locks(void); extern void debug_show_held_locks(struct task_struct *task); +extern void debug_show_all_lock_holders(void); extern void debug_check_no_locks_freed(const void *from, unsigned long len); extern void debug_check_no_locks_held(void); #else @@ -61,6 +62,10 @@ static inline void debug_show_held_locks(struct task_struct *task) { } +static inline void debug_show_all_lock_holders(void) +{ +} + static inline void debug_check_no_locks_freed(const void *from, unsigned long len) { diff --git a/kernel/hung_task.c b/kernel/hung_task.c index bb2354f73ded..18e22bbb714f 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -205,7 +205,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout) unlock: rcu_read_unlock(); if (hung_task_show_lock) - debug_show_all_locks(); + debug_show_all_lock_holders(); if (hung_task_show_all_bt) { hung_task_show_all_bt = false; diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 64a13eb56078..d06254108eb7 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -55,6 +55,7 @@ #include #include #include +#include #include @@ -6509,6 +6510,37 @@ void debug_show_all_locks(void) pr_warn("=============================================\n\n"); } EXPORT_SYMBOL_GPL(debug_show_all_locks); + +void debug_show_all_lock_holders(void) +{ + struct task_struct *g, *p; + + if (unlikely(!debug_locks)) { + pr_warn("INFO: lockdep is turned off.\n"); + return; + } + pr_warn("\nShowing all threads with locks held in the system:\n"); + + rcu_read_lock(); + for_each_process_thread(g, p) { + if (!p->lockdep_depth) + continue; + /* + * Assuming that the caller of this function is in a process + * context without any locks held, skip current thread which is + * holding only RCU read lock. + */ + if (p == current) + continue; + sched_show_task(p); + lockdep_print_held_locks(p); + touch_nmi_watchdog(); + touch_all_softlockup_watchdogs(); + } + rcu_read_unlock(); + pr_warn("\n"); + pr_warn("=============================================\n\n"); +} #endif /* -- 2.18.4