linux-kernel - Re: WARNING in ep_poll

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Date:   Tue, 08 Jan 2019 11:05:08 +0100
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: WARNING in ep_poll_callback

On 2019-01-08 07:00, Dmitry Vyukov wrote:
> On Tue, Jan 8, 2019 at 6:59 AM syzbot
> <syzbot+aea82bf9ee6ffd9a79d9@...kaller.appspotmail.com> wrote:
>> 
>> Hello,
>> 
>> syzbot found the following crash on:
>> 
>> HEAD commit:    139287cc2cc0 Add linux-next specific files for 
>> 20190108
>> git tree:       linux-next
>> console output: 
>> https://syzkaller.appspot.com/x/log.txt?x=16f563d7400000
>> kernel config:  
>> https://syzkaller.appspot.com/x/.config?x=1521b074ff5a5bdf
>> dashboard link: 
>> https://syzkaller.appspot.com/bug?extid=aea82bf9ee6ffd9a79d9
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>> 
>> Unfortunately, I don't have any reproducer for this crash yet.
> 
> Looks like caused by:
> 
> commit f92cacf118171208f62519d92502a8dd0341286d
> Author: Roman Penyaev
> Date:   Tue Jan 8 12:15:44 2019 +1100
> 
>     epoll: loosen irq safety in ep_poll_callback()

Yes, that was my wrong assumption that all drivers disable irqs before
waking up a wait queue.  Of course for drivers like fuse this is not
a case. Thanks for reporting.

The fix is simple: just remove wrong assert line :)  The whole logic
stays correct: epoll always locks rwlock with irqs disabled and by
itself does not call from interrupt, thus it is up to driver how to
call wake_up_locked(), because if driver does not handle any interrupts
it is safe on its side to take a simple spin_lock.

Patch is sent:
https://lore.kernel.org/lkml/20190108100121.20247-1-rpenyaev@suse.de

--
Roman