Re: WARNING in ep_poll_callback

[Roman Penyaev]

On 2019-01-08 07:00, Dmitry Vyukov wrote:
> On Tue, Jan 8, 2019 at 6:59 AM syzbot
> <syzbot+aea82bf9ee6ffd9a79d9@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxx

--
Roman