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

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Message-ID: <20110907183616.GH31726@redhat.com>
Date:	Wed, 7 Sep 2011 14:36:16 -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 02:27:49PM -0400, Konrad Rzeszutek Wilk wrote:
> On Wed, Sep 07, 2011 at 02:17:40PM -0400, Vivek Goyal wrote:
> > 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.
> 
> Ok, any thoughts on how to emulate it then perhaps? Mark each request after
> rq1 with WRITE_FUA? .. But then how long should the _FUA bit be set - perhaps
> until the rq1 has completed?

I think sender of the request need to wait for the completion of rq1
before issuing rq2 for emulating request ordering.

Thanks
Vivek
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