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

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Date:	Sat, 01 Sep 2007 14:09:43 -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>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [PATCH] Fix preemptible lazy mode bug

Zachary Amsden wrote:
> Do you agree it is better to be safe than sorry in this case?  The
> kind of bugs introduced by getting this wrong are really hard to find,
> and I would rather err on the side of an extra increment and decrement
> of preempt_count that causing a regression.

I think this patch is the direction we should go.  I this this would
work equally well for the other pv implementations; it would probably go
into the common lazy mode logic when we get around to doing it.

    J

diff -r b3fcc228c531 arch/i386/xen/enlighten.c
--- a/arch/i386/xen/enlighten.c	Mon Aug 20 14:20:15 2007 -0700
+++ b/arch/i386/xen/enlighten.c	Mon Aug 27 13:40:24 2007 -0700
@@ -250,6 +250,9 @@ static void xen_halt(void)
 
 static void xen_set_lazy_mode(enum paravirt_lazy_mode mode)
 {
+	if (preemptible() && mode == PARAVIRT_LAZY_FLUSH)
+		return;		/* nothing to flush with preempt on */
+
 	BUG_ON(preemptible());
 
 	switch (mode) {


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