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

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Date:	Thu, 06 Sep 2007 06:37:46 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
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>,
	Jeremy Fitzhardinge <jeremy@...p.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 22:46 -0700, 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.

Hi Zach,

	I don't think this patch does anything.  The flush is because we want
the just-completed "set_pte" to have immediate effect, so if preempt is
enabled we're already screwed because we can be moved between set_pte
and the arch_flush_lazy_mmu_mode() call.

Now, where's the problem caller?  By my reading or rc4, vmalloc faults
are fixed up before enabling interrupts.

Confused,
Rusty.


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