linux-kernel - Re: INFO: rcu detected stall in sys

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Message-ID: <20190312040829.GQ2217@ZenIV.linux.org.uk>
Date:   Tue, 12 Mar 2019 04:08:29 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     syzbot <syzbot+1505c80c74256c6118a5@...kaller.appspotmail.com>
Cc:     airlied@...ux.ie, akpm@...ux-foundation.org, amir73il@...il.com,
        chris@...is-wilson.co.uk, darrick.wong@...cle.com,
        david@...morbit.com, dri-devel@...ts.freedesktop.org,
        dvyukov@...gle.com, eparis@...hat.com, hannes@...xchg.org,
        hughd@...gle.com, intel-gfx@...ts.freedesktop.org, jack@...e.cz,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        jrdr.linux@...il.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mingo@...hat.com, mszeredi@...hat.com,
        penguin-kernel@...ove.sakura.ne.jp, peterz@...radead.org,
        rodrigo.vivi@...el.com, syzkaller-bugs@...glegroups.com,
        willy@...radead.org
Subject: Re: INFO: rcu detected stall in sys_sendfile64 (2)

On Mon, Mar 11, 2019 at 08:59:00PM -0700, syzbot wrote:
> syzbot has bisected this bug to:
> 
> commit 34e07e42c55aeaa78e93b057a6664e2ecde3fadb
> Author: Chris Wilson <chris@...is-wilson.co.uk>
> Date:   Thu Feb 8 10:54:48 2018 +0000
> 
>     drm/i915: Add missing kerneldoc for 'ent' in i915_driver_init_early
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=13220283200000
> start commit:   34e07e42 drm/i915: Add missing kerneldoc for 'ent' in i915..
> git tree:       upstream
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=10a20283200000
> console output: https://syzkaller.appspot.com/x/log.txt?x=17220283200000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=abc3dc9b7a900258
> dashboard link: https://syzkaller.appspot.com/bug?extid=1505c80c74256c6118a5
> userspace arch: amd64
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12c4dc28c00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15df4108c00000
> 
> Reported-by: syzbot+1505c80c74256c6118a5@...kaller.appspotmail.com
> Fixes: 34e07e42 ("drm/i915: Add missing kerneldoc for 'ent' in
> i915_driver_init_early")

Umm...  Might be a good idea to add some plausibility filters - it is,
in theory, possible that adding a line in a comment changes behaviour
(without compiler bugs, even - playing with __LINE__ is all it would
take), but the odds that it's _not_ a false positive are very low.