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

[Paul E. McKenney]

On Fri, May 07, 2021 at 10:42:48AM +0200, Dmitry Vyukov wrote:
> On Fri, May 7, 2021 at 10:38 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, May 06, 2021 at 02:34:27PM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    d2b6f8a1 Merge tag 'xfs-5.13-merge-3' of git://git.kernel...
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=123a56a5d00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=65c207250bba4efe
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=37fc8b84ffa2279d636d
> > > userspace arch: i386
> > >
> > > 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+37fc8b84ffa2279d636d@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > =============================
> > > WARNING: suspicious RCU usage
> > > 5.12.0-syzkaller #0 Not tainted
> > > -----------------------------
> > > kernel/sched/core.c:8304 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
> > > no locks held by syz-executor.4/10430.
> >
> > Looks like this is a concurrent fail?, if !debug_locks (as per the above)
> > then RCU_LOCKDEP_WARN() should not trigger.
> 
> FTR the log is here:
> https://syzkaller.appspot.com/text?tag=CrashLog&x=123a56a5d00000
> 
> It contains 2 intermixed reports:
> 
> [  289.896033][T10430] =============================
> [  289.896039][T10430] WARNING: suspicious RCU usage
> [  289.896046][T10430] 5.12.0-syzkaller #0 Not tainted
> [  289.898388][T13493] ======================================================
> [  289.898401][T13493] WARNING: possible circular locking dependency detected
> [  289.898410][T13493] 5.12.0-syzkaller #0 Not tainted
> [  289.898424][T13493] ------------------------------------------------------
> [  289.898432][T13493] syz-executor.0/13493 is trying to acquire lock:
> [  289.898448][T13493] ffff888019065ca0 (&bdev->bd_mutex){+.+.}-{3:3},
> at: del_gendisk+0x250/0x9e0

This "suspicious RCU usage" false positive looks to be addressed by this
commit in -rcu, slated for the v5.14 merge window:

81a5e05455d4 ("rcu: Reject RCU_LOCKDEP_WARN() false positives")

So I suggest ignoring the "suspicious RCU usage" report in favor of the
"possible circular locking dependency detected" report.

							Thanx, Paul