linux-kernel - Re: [syzbot] WARNING: suspicious RCU usage in copy_page

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Message-ID: <YGQlHvte0BeKx0uV@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Mar 2021 09:30:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+1a33233ccd8201ec2322@...kaller.appspotmail.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        LKML <linux-kernel@...r.kernel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] WARNING: suspicious RCU usage in copy_page_range

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@...kaller.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@...kaller.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.