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+88e4f02896967fe1a...@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+dde0cc33951735441...@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
> > <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 ;-)
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);
