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);