linux-kernel - Re: [patch] convert aio event reap to use atomic-op instead of spin

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Message-ID: <20070411195256.GD9602@mami.zabbo.net>
Date:	Wed, 11 Apr 2007 12:52:56 -0700
From:	Zach Brown <zach.brown@...cle.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Ken Chen <kenchen@...gle.com>, akpm@...ux-foundation.org,
	linux-aio@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] convert aio event reap to use atomic-op instead of spin_lock

> > I kept on getting requests from application developers who want that
> > feature.  My initial patch was dated back May 2004.
> 
> The right way to do it involves synchronization between the kernel side 
> io_getevents() and the userspace code pulling events out of the ring.  
> Alan Cox suggested embedding a futex in the shared memory region, but I 
> don't think anyone ever implemented that.

Yeah, I like the idea of futexes.

I'm worried that virtual aliasing spells doom for the current
home-brewed serialization that fs/aio.c is doing with the shared ring
head/tail accesses.  Am I worrying about nothing here?

> > I will look into this aside from this patch.
> 
> That's probably the case.  Also, any changes in this area *must* correctly 
> update the compat/incompat feature flags in the ring buffer header.  That 
> has been missed in the past...

Do you know of anyone using the current ring info ABI?

The *only* user I know of is the check of ctx->magic in libaio.

- z
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