linux-kernel - Re: Help with implementing some form of barriers in 3.0 kernels.

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Message-ID: <20110907181740.GG31726@redhat.com>
Date:	Wed, 7 Sep 2011 14:17:40 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	hch@...radead.org, Jeremy Fitzhardinge <jeremy@...p.org>,
	jbeulich@...ell.com, linux-kernel@...r.kernel.org,
	JBeulich@...e.com
Subject: Re: Help with implementing some form of barriers in 3.0 kernels.

On Wed, Sep 07, 2011 at 01:48:32PM -0400, Konrad Rzeszutek Wilk wrote:
> Hey Christoph,
> 
> I was wondering what you think is the proper way of implementing a
> backend to support the 'barrier' type requests? We have this issue were
> there are 2.6.36 type guests that still use barriers and we would like
> to support them properly. But in 3.0 there are no barriers - hence
> the question whether WRITE_fLUSH_FUA would be equal to WRITE_BARRIER?

I think WRITE_FLUSH_FUA is not same as WRITE_BARRIER. Because it does
not ensure request ordering. A request rq2 which is issued after rq1 (with
WRITE_flush_FUA), can still finish before rq1. In the past WRITE_BARRIER
would not allow that.

So AFAIK, WRITE_flush_fua is not WRITE_BARRIER.

Thanks
Vivek
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