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Message-ID: <46CE8069.9070404@goop.org>
Date: Thu, 23 Aug 2007 23:53:29 -0700
From: Jeremy Fitzhardinge <jeremy@...p.org>
To: Zachary Amsden <zach@...are.com>
CC: 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,
Rusty Russell <rusty@...tcorp.com.au>,
Virtualization Mailing List <virtualization@...ts.osdl.org>,
Andi Kleen <ak@...e.de>
Subject: Re: [PATCH] Fix preemptible lazy mode bug
Zachary Amsden wrote:
> I recently sent off a fix for lazy vmalloc faults which can happen
> under paravirt when lazy mode is enabled. Unfortunately, I jumped the
> gun a bit on fixing this. I neglected to notice that since the new
> call to flush the MMU update queue is called from the page fault
> handler, it can be pre-empted. Both VMI and Xen use per-cpu variables
> to track lazy mode state, as all previous calls to set, disable, or
> flush lazy mode happened from a non-preemptable state.
Hm. Doing any kind of lazy-state operation with preemption enabled is
fundamentally meaningless. How does it get into a preemptable state
with a lazy mode enabled now? If a sequence of code with preempt
disabled touches a missing vmalloc mapping, it gets a fault to fix up
the mapping, and the fault handler can end up preempting the thread?
That sounds like a larger bug than just paravirt lazy mode problems.
J
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