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From: Paul Durrant <Paul.Durrant@citrix.com>
To: Anthony Perard <anthony.perard@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: [Xen-devel] [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore
Date: Thu, 22 Aug 2019 13:21:20 +0000
Message-ID: <8e3dd40d4d3d4ff4af42a0feacae5ede@AMSPEX02CL03.citrite.net> (raw)
In-Reply-To: <20190822124427.GL1289@perard.uk.xensource.com>

> -----Original Message-----
> From: Anthony PERARD <anthony.perard@citrix.com>
> Sent: 22 August 2019 14:18
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: qemu-devel@nongnu.org; Stefano Stabellini <sstabellini@kernel.org>; xen-devel@lists.xenproject.org
> Subject: Re: [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore
> 
> On Thu, Aug 22, 2019 at 12:25:44PM +0100, Paul Durrant wrote:
> > > From: Anthony PERARD <anthony.perard@citrix.com>
> > > Sent: 22 August 2019 12:18
> > >
> > > On Thu, Aug 22, 2019 at 11:36:32AM +0100, Paul Durrant wrote:
> > > > But, now I look at the code again without your patch applied I don't actually see the problem it
> is
> > > trying to fix. The functions xen_device_[back|front]end_set_state return early if the state being
> set
> > > matches the existing state and hence never get to the line where the state is written to xenstore.
> > >
> > > Let's see:
> > >     * step 1 (initial states in xenstore and QEMU)
> > >         xenstore/frontend/state = 4
> > >         xendev->frontend_state = 4
> > >     * step 2 (frontend changes state in xenstore)
> > >         xenstore/frontend/state = 5
> > >     * step 3 (watch event received by QEMU)
> > >         xen_device_frontend_changed()
> > >             state = read(xenstore/frontend/state) (state=5)
> > >             xen_device_frontend_set_state(state)
> > >                 xendev->frontend_state != state  (4!=5)
> > >                     xendev->frontend_state = state
> > >                     xenstore/frontend/state = state
> > >     * step 4
> > >         # watch event triggers xen_device_frontend_changed() again but
> > >         # this time xendev->frontend_state == xenstore/frontend_state
> > >
> > > This is how QEMU writes to xenstore an identical value.
> > >
> > > That behavior might be an issue if the frontend changes the value after
> > > QEMU have read it but before QEMU writes it again.
> >
> > Ah, ok, so the problem is actually limited to frontend state because that is written by both
> frontend and backend, so whether QEMU writes an updated frontend state to xenstore needs to be
> controlled. It's only called in two places xen_device_frontend_changed() and xen_device_realize(). The
> write to xenstore should be avoided in the former case, but not the latter. So adding a 'publish'
> boolean and using that to determine whether the write to xenstore is done seems like the right
> approach. But I don't think any change is needed to xen_device_backend_set_online() or
> xen_device_backend_set_state(), is it?
> 
> I guess it's not that much of a issue for backend_set_*(), the double
> write would only happen when the toolstack try to tear down the backend,
> so it would happen only once.
> 
> Alright, I'll only change frontend_set_state() and use 'publish'.

Thanks :-)

  Paul

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Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20190821092020.17952-1-anthony.perard@citrix.com>
2019-08-21  9:20 ` [Xen-devel] [PATCH 1/2] xen-bus: Fix backend state transition on device reset Anthony PERARD
2019-08-21  9:36   ` Paul Durrant
2019-08-22  9:50     ` Anthony PERARD
2019-08-22  9:59       ` Paul Durrant
2019-08-22 15:01         ` Anthony PERARD
2019-08-21  9:20 ` [Xen-devel] [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore Anthony PERARD
2019-08-21 15:40   ` Paul Durrant
2019-08-22 10:21     ` Anthony PERARD
2019-08-22 10:36       ` Paul Durrant
2019-08-22 11:17         ` Anthony PERARD
2019-08-22 11:25           ` Paul Durrant
2019-08-22 13:18             ` Anthony PERARD
2019-08-22 13:21               ` Paul Durrant [this message]

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