linux-kernel - Re: [PATCH 2/8] aio_poll_wake(): don't set ->woken if we ignore the wakeup

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Date:   Thu, 7 Mar 2019 02:18:44 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        Jason Baron <jbaron@...mai.com>, kgraul@...ux.ibm.com,
        ktkhai@...tuozzo.com, kyeongdon.kim@....com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>, pabeni@...hat.com,
        syzkaller-bugs@...glegroups.com, xiyou.wangcong@...il.com,
        Christoph Hellwig <hch@....de>,
        zhengbin <zhengbin13@...wei.com>, bcrl@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        houtao1@...wei.com, yi.zhang@...wei.com
Subject: Re: [PATCH 2/8] aio_poll_wake(): don't set ->woken if we ignore the
 wakeup

On Thu, Mar 07, 2019 at 12:03:10AM +0000, Al Viro wrote:
> From: Al Viro <viro@...iv.linux.org.uk>
> 
> In case of early wakeups, aio_poll() assumes that aio_poll_complete()
> has either already happened or is imminent.  In that case we do not
> want to put iocb on the list of cancellables.  However, ignored
> wakeups need to be treated as if wakeup has not happened at all.
> Trivially fixed by having aio_poll_wake() set ->woken only after
> it's committed to taking iocb out of the waitqueue.
> 
> Spotted-by: zhengbin <zhengbin13@...wei.com>
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>

... and unfortunately it's worse than just that - what both of us
have missed is that one could have non-specific wakep + schedule_work +
aio_poll_complete_work() rechecking ->poll(), seeing nothing of
interest and reinserting into queue.  All before vfs_poll() manages
to return into aio_poll().  The window is harder to hit, but it's
still there, with exact same "failed to add to cancel list" kind of bug
if we do hit it ;-/