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

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Date:   Thu, 2 Aug 2018 18:16:48 +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 05:08:38PM +0100, Al Viro wrote:
> 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...

But we'll never re-add an iocb once it has been removed from the
waitqueue.