Date: Tue, 22 Feb 2011 18:13:41 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Jan Beulich <JBeulich@novell.com>, Ian Campbell <Ian.Campbell@citrix.com>,
        Andi Kleen <andi@firstfloor.org>, Hugh Dickins <hughd@google.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Xen-devel@lists.xensource.com" <Xen-devel@lists.xensource.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Johannes Weiner <jweiner@redhat.com>,
        Larry Woodman <lwoodman@redhat.com>, Rik van Riel <riel@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] fix pgd_lock deadlock
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 <4D63D4CD020000780003320A@vpn.id2.novell.com>
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On Tue, Feb 22, 2011 at 09:08:54AM -0800, Jeremy Fitzhardinge wrote:
> On 02/22/2011 06:34 AM, Andrea Arcangeli wrote:
> > On Tue, Feb 22, 2011 at 02:22:53PM +0000, Jan Beulich wrote:
> >> If everyone's okay with putting a couple of "if (xen_pv_domain())"
> >> into mm/fault.c - sure. I would have thought that this wouldn't be
> >> liked, hence the suggestion to make this depend on seeing the
> >> backlink be non-NULL.
> > I prefer the if (xen_pv_domain) so it also gets optimized away
> > at build time when CONFIG_XEN=n. I think it's also more self
> > explanatory to read.
> 
> Eh, very not keen about that.  I'd only consider it as a last resort.
> 
> In practice CONFIG_XEN is always enabled in distros, so the conditional
> would always be done at runtime.

That's ok, it's still better than setting a page filed IMHO. As the
page flag would be set conditional to xen_pv_domain() anyway. I
wouldn't like to make things more complicated than they already are,
this will make it more explicit why that spinlock is needed too.
Unless we foresee other hypervisors using a generic functionality we
don't need one.
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