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=-12.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 A30F6C433B4 for ; Mon, 5 Apr 2021 17:27:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6EF4C613B5 for ; Mon, 5 Apr 2021 17:27:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238032AbhDER2B (ORCPT ); Mon, 5 Apr 2021 13:28:01 -0400 Received: from mail.kernel.org ([198.145.29.99]:52070 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237952AbhDER2A (ORCPT ); Mon, 5 Apr 2021 13:28:00 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 05D33613B3; Mon, 5 Apr 2021 17:27:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1617643673; bh=52mH7ZB5aTWVBvqpavWjW4cw9/KHCXamIgO2r2KxjRI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=TrsbW+Co0EabJ7WhEAe6l64j+go+N+eiAL9DTuzRrmkSN+LNIxzoPE2/5vDa4EmJM o2og0VpoebfDcYVjTF8XqQFgp0GHQClV+txqIfr5HM9ixxBhg2PBMnnMoQo9bJKJy1 WnbRw/lvrXF9Xe5deBrpVMht/jdIfaV8tgBDI1NJXyfXE2KghvQv+vRHw6TqEeaQSG ybRiSow2gL1KM280U6fANMlDP6ZNiNK+ecmMLRWK9kuZ2b1X/ZwFVgBJBPrksIG6DU djc7WMadboFmYz8BUF1pmKRXtJudMy0R/TOwUJl64qYCYIGv5stDfX+cCMLW6j3qo8 5w6Oj0tkUc6Pg== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id CEF0935225D5; Mon, 5 Apr 2021 10:27:52 -0700 (PDT) Date: Mon, 5 Apr 2021 10:27:52 -0700 From: "Paul E. McKenney" To: Boqun Feng Cc: Matthew Wilcox , Thomas Gleixner , syzbot , john.stultz@linaro.org, linux-kernel@vger.kernel.org, sboyd@kernel.org, syzkaller-bugs@googlegroups.com, Peter Zijlstra Subject: Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64 Message-ID: <20210405172752.GK2696@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <0000000000000e025b05bf2a430b@google.com> <87mtud4wfi.ffs@nanos.tec.linutronix.de> <20210404214030.GB2696@paulmck-ThinkPad-P72> <20210405030855.GG2531743@casper.infradead.org> <20210405040125.GF2696@paulmck-ThinkPad-P72> <20210405043038.GA31091@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 05, 2021 at 01:23:30PM +0800, Boqun Feng wrote: > On Sun, Apr 04, 2021 at 09:30:38PM -0700, Paul E. McKenney wrote: > > On Sun, Apr 04, 2021 at 09:01:25PM -0700, Paul E. McKenney wrote: > > > On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote: > > > > On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote: > > > > > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote: > > > > > > On Sun, Apr 04 2021 at 12:05, syzbot wrote: > > > > > > > > > > > > Cc + ... > > > > > > > > > > And a couple more... > > > > > > > > > > > > Hello, > > > > > > > > > > > > > > syzbot found the following issue on: > > > > > > > > > > > > > > HEAD commit: 5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co.. > > > > > > > git tree: upstream > > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000 > > > > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=78ef1d159159890 > > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d > > > > > > > > > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > > > > > > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > > > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com > > > > > > > > > > > > > > ============================= > > > > > > > WARNING: suspicious RCU usage > > > > > > > 5.12.0-rc5-syzkaller #0 Not tainted > > > > > > > ----------------------------- > > > > > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section! > > > > > > > > > > > > > > other info that might help us debug this: > > > > > > > > > > > > > > > > > > > > > rcu_scheduler_active = 2, debug_locks = 0 > > > > > > > 3 locks held by syz-executor.4/8418: > > > > > > > #0: > > > > > > > ffff8880751d2b28 > > > > > > > ( > > > > > > > &p->pi_lock > > > > > > > ){-.-.}-{2:2} > > > > > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345 > > > > > > > #1: > > > > > > > ffff8880b9d35258 > > > > > > > ( > > > > > > > &rq->lock > > > > > > > ){-.-.}-{2:2} > > > > > > > , at: rq_lock kernel/sched/sched.h:1321 [inline] > > > > > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline] > > > > > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464 > > > > > > > #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807 > > > > > > > > > > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com > > > > > in that rcu_sleep_check() sees an RCU lock held, but the later call to > > > > > lockdep_print_held_locks() does not. Did something change recently that > > > > > could let the ->lockdep_depth counter get out of sync with the actual > > > > > number of locks held? > > > > > > > > Dmitri had a different theory here: > > > > > > > > https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ > > > > > > There is always room for more than one bug. ;-) > > > > > > He says "one-off false positives". I was afraid of that... > > > > And both the examples I have been copied on today are consistent with > > debug_locks getting zeroed (e.g., via a call to __debug_locks_off()) > > in the midst of a call to rcu_sleep_check(). But I would expect to see > > a panic or another splat if that were to happen. > > > > Dmitry's example did have an additional splat, but I would expect the > > RCU-related one to come second. Again, there is always room for more > > than one bug. > > > > On the other hand, there are a lot more callers to debug_locks_off() > > than there were last I looked into this. And both of these splats > > are consistent with an interrupt in the middle of rcu_sleep_check(), > > and that interrupt's handler invoking debug_locks_off(), but without > > printing anything to the console. Does that sequence of events ring a > > bell for anyone? > > > > If this is the new normal, I could make RCU_LOCKDEP_WARN() recheck > > debug_lockdep_rcu_enabled() after evaluating the condition, but with > > a memory barrier immediately before the recheck. But I am not at all > > excited by doing this on speculation. Especially given that doing > > so might be covering up some other bug. > > > > Just check the original console log and find: > > [ 356.696686][ T8418] ============================= > [ 356.696692][ T8418] WARNING: suspicious RCU usage > [ 356.700193][T14782] ==================================== > [ 356.704548][ T8418] 5.12.0-rc5-syzkaller #0 Not tainted > [ 356.729981][ T8418] ----------------------------- > [ 356.732473][T14782] WARNING: iou-sqp-14780/14782 still has locks held! > > , so there are two warnnings here, one is from lockdep_rcu_suspisous() > and the other is from print_held_locks_bug(). I think this is what > happened: > > in RCU_LOCKDEP_WARN(): > > if (debug_lockdep_rcu_enabled() // this is true and at this time debug_locks = 1 > > // lockdep detects a lock bug, set debug_locks = 0 > > && !__warned // true > && (c)) // "c" is a lock_is_held(), which will always returns true if debug_locks == 0! > > the cause of the problem is that RCU_LOCKDEP_WARN() in fact read > debug_locks twice and get different values. > > But if you change the ordering of two reads, probably can avoid the > problem: > > First read: > lock_is_held(); // true if 1) lock is really held or 2) lockdep is off > > Second read: > debug_lockdep_rcu_enabled(); // if lockdep is not off, we know > // that the first read got correct > // value, otherwise we just ignore > // the first read, because either > // there is a bug reported between > // two reads, or lockdep is already > // off when the first read happens. > > So maybe something below: > > Regards, > Boqun > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index bd04f722714f..d11bee5d9347 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void) > #define RCU_LOCKDEP_WARN(c, s) \ > do { \ > static bool __section(".data.unlikely") __warned; \ > - if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \ > + if ((c) && debug_lockdep_rcu_enabled() && !__warned) { \ > __warned = true; \ > lockdep_rcu_suspicious(__FILE__, __LINE__, s); \ > } \ Good point -- if we check debug_lockdep_rcu_enabled() after the condition, then we will reject false positives in cases where debug_locks was switched to zero out from under us. However, we do need ordering. The "c" usually contains lock_is_held(), which also checks debug_locks, but from some other translation unit. Back in the day, the translation-unit boundaries would provide the needed ordering, but LTO... In addition, the "debug_locks = 0" was originally supposed to be a hint that the report might be a false positive. It is clear that this needs to be made explicit. Taking all this together, how about the following? (The intent is that the changes to lockdep_rcu_suspicious() will be in a separate commit.) Thanx, Paul ------------------------------------------------------------------------ diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 9455476..1199ffd 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void) #define RCU_LOCKDEP_WARN(c, s) \ do { \ static bool __section(".data.unlikely") __warned; \ - if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \ + if ((c) && debug_lockdep_rcu_enabled() && !__warned) { \ __warned = true; \ lockdep_rcu_suspicious(__FILE__, __LINE__, s); \ } \ diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index c6d0c1dc..80065cd 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -6387,6 +6387,7 @@ asmlinkage __visible void lockdep_sys_exit(void) void lockdep_rcu_suspicious(const char *file, const int line, const char *s) { struct task_struct *curr = current; + int dl = READ_ONCE(debug_locks); /* Note: the following can be executed concurrently, so be careful. */ pr_warn("\n"); @@ -6396,11 +6397,12 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s) pr_warn("-----------------------------\n"); pr_warn("%s:%d %s!\n", file, line, s); pr_warn("\nother info that might help us debug this:\n\n"); - pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n", + pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n%s", !rcu_lockdep_current_cpu_online() ? "RCU used illegally from offline CPU!\n" : "", - rcu_scheduler_active, debug_locks); + rcu_scheduler_active, dl, + dl ? "" : "Possible false positive due to lockdep disabling via debug_locks = 0\n"); /* * If a CPU is in the RCU-free window in idle (ie: in the section diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index b95ae86..dd94a60 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -277,7 +277,7 @@ EXPORT_SYMBOL_GPL(rcu_callback_map); noinstr int notrace debug_lockdep_rcu_enabled(void) { - return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks && + return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && READ_ONCE(debug_locks) && current->lockdep_recursion == 0; } EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);