Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

From: Boqun Feng
Date: Mon Apr 05 2021 - 19:43:21 EST

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In reply to: Paul E. McKenney: "Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64"
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On Mon, Apr 05, 2021 at 04:38:07PM -0700, Paul E. McKenney wrote:
> On Tue, Apr 06, 2021 at 07:25:44AM +0800, Boqun Feng wrote:
> > On Mon, Apr 05, 2021 at 10:27:52AM -0700, Paul E. McKenney wrote:
> > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > > > > > > >
> > > > > > > > > > =============================
> > > > > > > > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > > > > > 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.)
> >
> > Looks good to me ;-)
>
> Whew! May I add your Reviewed-by?
>

Of course ;-)

Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>

Regards,
Boqun

> Thanx, Paul
>
> > Regards,
> > Boqun
> >
> > > 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);

Next message: Chuck Lever III: "Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs"
Previous message: Waiman Long: "[PATCH v4] sched/debug: Use sched_debug_lock to serialize use of cgroup_path[] only"
In reply to: Paul E. McKenney: "Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64"
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