linux-kernel - Re: [PATCH 3/4] aio: implement IOCB_CMD

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Date:   Thu, 2 Aug 2018 18:08:16 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, Avi Kivity <avi@...lladb.com>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] aio: implement IOCB_CMD_POLL

On Thu, Aug 02, 2018 at 05:00:32PM +0100, Al Viro wrote:
> BTW, what happens if we insert into one queue and immediately get
> woken up, even before the damn thing gets to the end of ->poll(),
> which proceeds to call poll_wait() again (on another queue)?
> AFAICS, apt.error will be set by the second callback and completely
> ignored.  And so will the return value of ->poll()...
> 
> Sigh...  Analysis of that thing is bloody painful, mostly because
> it's hard to describe the state...

That's the problem with the ->poll interface.  We call it, then
have magic happen underneath where it might or might not get added
to one (or more if we didn't exclude that) waitqueues, and might
have actually been worken before return.  I can't really think of
a good way to do that entirely sanely.

Best I can think of is to only allow using file ops that do keyed
wakeups and rely on the keyed wakeups alone.  I've started coming
up with a version of that, but it won't be until tomorrow at least
that I can post it.