From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-fsdevel-owner@vger.kernel.org>
Received: from zeniv.linux.org.uk ([195.92.253.2]:35576 "EHLO
        ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1726848AbeHBSA2 (ORCPT
        <rfc822;linux-fsdevel@vger.kernel.org>);
        Thu, 2 Aug 2018 14:00:28 -0400
Date: Thu, 2 Aug 2018 17:08:38 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Christoph Hellwig <hch@lst.de>
Cc: Avi Kivity <avi@scylladb.com>, linux-aio@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/4] aio: implement IOCB_CMD_POLL
Message-ID: <20180802160837.GZ30522@ZenIV.linux.org.uk>
References: <20180730071544.23998-1-hch@lst.de>
 <20180730071544.23998-4-hch@lst.de>
 <20180802002121.GU30522@ZenIV.linux.org.uk>
 <20180802092234.GA13797@lst.de>
 <20180802160032.GY30522@ZenIV.linux.org.uk>
 <20180802160816.GA9447@lst.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180802160816.GA9447@lst.de>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

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...