linux-kernel - Re: [PATCH] Fix preemptible lazy mode bug

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Message-Id: <1188043030.20041.75.camel@localhost.localdomain>
Date:	Sat, 25 Aug 2007 21:57:10 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Zachary Amsden <zach@...are.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linus Torvalds <torvalds@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Chris Wright <chrisw@...s-sol.org>, stable@...nel.org,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [PATCH] Fix preemptible lazy mode bug

On Thu, 2007-08-23 at 23:59 -0700, Zachary Amsden wrote:
> Jeremy Fitzhardinge wrote:
> > Hm.  Doing any kind of lazy-state operation with preemption enabled is
> > fundamentally meaningless.  How does it get into a preemptable state
> >   
> 
> Agree 100%.  It is the lazy mode flush that might happen when preempt is 
> enabled, but lazy mode is disabled.  In that case, the code relies on 
> per-cpu variables, which is a bad thing to do in preemtible code.  This 
> can happen in the current code path.

Frankly, we should hoist the per-cpu state into generic paravirt code,
get rid of the FLUSH "state" and only call the lazy_mode hooks when
actually entering or exiting a lazy mode.

The only reason lguest doesn't use a per-cpu var is that guests are
currently UP only.  If that were fixed, we'd have identical VMI, Xen and
lguest lazy state handing.

Cheers,
Rusty.


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