From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: syzbot <syzbot+1a33233ccd8201ec2322@syzkaller.appspotmail.com>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Jens Axboe <axboe@kernel.dk>,
Christian Brauner <christian@brauner.io>,
LKML <linux-kernel@vger.kernel.org>,
Shakeel Butt <shakeelb@google.com>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: [syzbot] WARNING: suspicious RCU usage in copy_page_range
Date: Tue, 6 Apr 2021 09:41:10 +0200 [thread overview]
Message-ID: <YGwQlnpKBbUrGaZm@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CACT4Y+b1NsdnC1hqk54Y8zEs7r3y7+EnAqbG1eBmuhji_bfFqw@mail.gmail.com>
On Wed, Mar 31, 2021 at 11:57:23AM +0200, Dmitry Vyukov wrote:
> On Wed, Mar 31, 2021 at 9:31 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Mar 31, 2021 at 08:11:38AM +0200, Dmitry Vyukov wrote:
> > > On Wed, Mar 31, 2021 at 12:26 AM syzbot
> > > <syzbot+1a33233ccd8201ec2322@syzkaller.appspotmail.com> wrote:
> > > >
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit: db24726b Merge tag 'integrity-v5.12-fix' of git://git.kern..
> > > > git tree: upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=16c16b7cd00000
> > > > kernel config: https://syzkaller.appspot.com/x/.config?x=daeff30c2474a60f
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=1a33233ccd8201ec2322
> > > >
> > > > 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+1a33233ccd8201ec2322@syzkaller.appspotmail.com
> > >
> > > I think this is a LOCKDEP issue. +LOCKDEP maintainers.
> > >
> > > Another bug happened on another thread ("WARNING: possible circular
> > > locking dependency detected"). Lockdep disabled lock tracking
> > > ("debug_locks = 0" in the report), which probably made it miss
> > > rcu_unlock somewhere, but it did not turn off reporting yet and
> > > produced the false positive first.
> > >
> > > I think if LOCKDEP disables lock tracking, it must also disable
> > > reporting of issues that require lock tracking. That would avoid false
> > > positives.
> >
> > Still early and brain hasn't really booted yet, but features that
> > require lock tracking are supposed to check debug_locks.
> >
> > And afaict debug_lockdep_rcu_enabled(), which is called by
> > RCU_LOCKDEP_WARN(), which is called by rcu_sleep_check() does just that.
>
> Right... yet it somehow happens.
> Looking at a dozen of reports, all with 2 concurrent lockdep splats
> and "debug_locks = 0" in the report, I am pretty sure there is some
> kind of race in lockdep.
Aah, concurrent splats. Yes, that was per design. The theory was that
concurrent splats are rare and this is much simpler code.
next prev parent reply other threads:[~2021-04-06 7:41 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-30 22:26 [syzbot] WARNING: suspicious RCU usage in copy_page_range syzbot
2021-03-31 6:11 ` Dmitry Vyukov
2021-03-31 7:30 ` Peter Zijlstra
2021-03-31 9:57 ` Dmitry Vyukov
2021-04-06 7:41 ` Peter Zijlstra [this message]
2021-04-13 18:12 ` Dmitry Vyukov
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