netdev - Re: [PATCH 06/28] aio: implement IOCB_CMD

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Message-ID: <20180322165255.GI30522@ZenIV.linux.org.uk>
Date:   Thu, 22 Mar 2018 16:52:55 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Avi Kivity <avi@...lladb.com>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/28] aio: implement IOCB_CMD_POLL

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...