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

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Date:   Thu, 2 Aug 2018 22:48:18 +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 06:16:48PM +0200, Christoph Hellwig wrote:
> 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.

Umm...  Frankly, I wonder if the right approach is to declare that if
wakeup has happened at all, submit gives up any responsibility.

IOW, have aio_poll
	* create iocb, feed to ->poll()
	* check if ->head is NULL; if it is, the mask we've got is
*all* we are going to get; complete and bugger off.
	* lock ioctx
	* insert the sucker into ->active_reqs
	* lock the queue
	* check if wakeup has already happened.
	* if it has - go away, it's submitted
	* check apt.error; if set, fail with -EINVAL.
	* check the mask; if nothing of interest is in there, go away
	* dequeue, unlist, complete and bugger off

On cancel side
	* lock queue
	* mark it cancelled
	* removed from queue if it was there
	* unlock queue

On wakeup
	* mark it woken
	* complete or schedule completion, which would be where we check
if it's marked cancelled - both before vfs_poll() (to avoid calling it
if already set) and after (treating cancel during vfs_poll() as "complete
and bugger off").

Comments?