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

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Date:	Thu, 23 Aug 2007 22:46:48 -0700
From:	Zachary Amsden <zach@...are.com>
To:	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,
	Rusty Russell <rusty@...tcorp.com.au>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Andi Kleen <ak@...e.de>
Subject: [PATCH] Fix preemptible lazy mode bug

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.

I have no idea how to convincingly produce the problem, as generating a 
kernel pre-emption at the required point is, um, difficult, but it is 
most certainly a real possibility, and potentially more likely than the 
bug I fixed originally.

Rusty, you may have to modify lguest code if you use lazy mode and rely 
on per-cpu variables during the callout for paravirt_ops.set_lazy_mode.

I have tested as best as I can, and am trying to write a suite destined 
for LTP which will help catch and debug these issues.

Zach

View attachment "i386-paravirt-preempt-fix.patch" of type "text/x-patch" (2257 bytes)