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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Fri, 7 May 2021 10:42:48 +0200
Message-ID: <CACT4Y+aaGyfiEgbKca-6gourQjtwVMHuwUcb30QJPOF1LhLcxQ@mail.gmail.com>
Subject: Re: [syzbot] WARNING: suspicious RCU usage in get_signal
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     syzbot <syzbot+37fc8b84ffa2279d636d@syzkaller.appspotmail.com>,
        Jens Axboe <axboe@kernel.dk>,
        Christian Brauner <christian@brauner.io>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Peter Collingbourne <pcc@google.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul McKenney <paulmck@kernel.org>
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On Fri, May 7, 2021 at 10:38 AM Peter Zijlstra <peterz@infradead.org> 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@syzkaller.appspotmail.com
> >
> > =============================
> > 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