Re: [PATCH 06/28] aio: implement IOCB_CMD_POLL

[Al Viro]

On Wed, Mar 21, 2018 at 08:40:10AM +0100, Christoph Hellwig wrote:
> Simple one-shot poll through the io_submit() interface.  To poll for
> a file descriptor the application should submit an iocb of type
> IOCB_CMD_POLL.  It will poll the fd for the events specified in the
> the first 32 bits of the aio_buf field of the iocb.
> 
> Unlike poll or epoll without EPOLLONESHOT this interface always works
> in one shot mode, that is once the iocb is completed, it will have to be
> resubmitted.

AFAICS, your wakeup can race with io_cancel(), leading to double fput().
You are checking the "somebody had committed itself to cancelling that
thing" bit outside of ->ctx_lock on the wakeup side, and I don't see
anything to prevent both getting to __aio_poll_complete() on the same
iocb, with obvious results.

I might be missing something subtle in there, but then it would be nice to
have it covered in commit message...