linux-kernel - Re: [PATCH -mm 4/5][AIO] - AIO completion signal notification

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Message-ID: <20061129142516.58eb38df@frecb000686>
Date:	Wed, 29 Nov 2006 14:25:16 +0100
From:	Sébastien Dugué <sebastien.dugue@...l.net>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-aio <linux-aio@...ck.org>, Andrew Morton <akpm@...l.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Zach Brown <zach.brown@...cle.com>,
	Badari Pulavarty <pbadari@...ibm.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Jean Pierre Dion <jean-pierre.dion@...l.net>
Subject: Re: [PATCH -mm 4/5][AIO] - AIO completion signal notification

On Wed, 29 Nov 2006 06:33:35 -0500, Jakub Jelinek <jakub@...hat.com> wrote:

> On Wed, Nov 29, 2006 at 11:33:01AM +0100, S?bastien Dugu? wrote:
> >                       AIO completion signal notification
> > 
> >   The current 2.6 kernel does not support notification of user space via
> > an RT signal upon an asynchronous IO completion. The POSIX specification
> > states that when an AIO request completes, a signal can be delivered to
> > the application as notification.
> > 
> >   This patch adds a struct sigevent *aio_sigeventp to the iocb.
> > The relevant fields (pid, signal number and value) are stored in the kiocb
> > for use when the request completes.
> > 
> >   That sigevent structure is filled by the application as part of the AIO
> > request preparation. Upon request completion, the kernel notifies the
> > application using those sigevent parameters. If SIGEV_NONE has been specified,
> > then the old behaviour is retained and the application must rely on polling
> > the completion queue using io_getevents().
> 
> Well, from what I see applications must rely on polling the completion
> queue using io_getevents() in any case, isn't that the only way how to free
> the kernel resources associated with the AIO request, even if it uses
> SIGEV_SIGNAL or thread notification?

  Well, applications do not need to do any polling on the queue anymore.
io_getevents() needs to be called only once when the signal has been received,
either from a signal handler or from a thread blocking on the signal.

> aio_error/aio_return/aio_suspend
> will still need to io_getevents,

  Right, but only once.

> the only important difference with this
> patch is that a) the polling doesn't need to be asynchronous (i.e. have
> a special thread which just loops doing io_getevents)

  Yes, no more polling loop and I think this makes a big difference.

> b) it doesn't need to care about notification itself.
> 

  Uh! what do you mean here?

  Sébastien.
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