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

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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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