Re: possible deadlock in __io_queue_deferred

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Aug 11, 2020 at 08:21:12AM -0600, Jens Axboe wrote:
> On 8/11/20 8:00 AM, Stefano Garzarella wrote:
> > On Mon, Aug 10, 2020 at 09:55:17AM -0600, Jens Axboe wrote:
> >> On 8/10/20 9:36 AM, syzbot wrote:
> >>> Hello,
> >>>
> >>> syzbot found the following issue on:
> >>>
> >>> HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
> >>> git tree:       upstream
> >>> console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
> >>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
> >>> dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
> >>> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> >>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
> >>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
> >>>
> >>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >>> Reported-by: syzbot+996f91b6ec3812c48042@syzkaller.appspotmail.com
> >>
> >> Thanks, the below should fix this one.
> > 
> > Yeah, it seems right to me, since only __io_queue_deferred() (invoked by
> > io_commit_cqring()) can be called with 'completion_lock' held.
> 
> Right
> 
> > Just out of curiosity, while exploring the code I noticed that we call
> > io_commit_cqring() always with the 'completion_lock' held, except in the
> > io_poll_* functions.
> > 
> > That's because then there can't be any concurrency?
> 
> Do you mean the iopoll functions? Because we're definitely holding it
> for the io_poll_* functions.

Right, the only one seems io_iopoll_complete().

So, IIUC, in this case we are actively polling the level below,
so there shouldn't be any asynchronous events, is it right?

Thanks,
Stefano