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

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Date:   Thu, 2 Aug 2018 17:08:38 +0100
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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] aio: implement IOCB_CMD_POLL

On Thu, Aug 02, 2018 at 06:08:16PM +0200, Christoph Hellwig wrote:
> 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.

What does it buy you?  You still have to deal with trylock failures
in wakeup...