Date:   Mon, 5 Apr 2021 10:27:52 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        syzbot <syzbot+88e4f02896967fe1ab0d@...kaller.appspotmail.com>,
        john.stultz@...aro.org, linux-kernel@...r.kernel.org,
        sboyd@...nel.org, syzkaller-bugs@...glegroups.com,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

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@...kaller.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@...kaller.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
> 	<interrupted>
> 	// lockdep detects a lock bug, set debug_locks = 0
> 	<swicth back>
> 	    && !__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);

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