linux-kernel - [15/48] KVM guest: prevent tracing recursion with kvmclock

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Message-Id: <20120116184517.042963719@clark.kroah.org>
Date:	Mon, 16 Jan 2012 10:44:42 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Avi Kivity <avi@...hat.com>
Subject: [15/48] KVM guest: prevent tracing recursion with kvmclock

3.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Avi Kivity <avi@...hat.com>

(cherry picked from commit 95ef1e52922cf75b1ea2eae54ef886f2cc47eecb)

Prevent tracing of preempt_disable() in get_cpu_var() in
kvm_clock_read(). When CONFIG_DEBUG_PREEMPT is enabled,
preempt_disable/enable() are traced and this causes the function_graph
tracer to go into an infinite recursion. By open coding the
preempt_disable() around the get_cpu_var(), we can use the notrace
version which prevents preempt_disable/enable() from being traced and
prevents the recursion.

Based on a similar patch for Xen from Jeremy Fitzhardinge.

Tested-by: Gleb Natapov <gleb@...hat.com>
Acked-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Avi Kivity <avi@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
---
 arch/x86/kernel/kvmclock.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -74,9 +74,10 @@ static cycle_t kvm_clock_read(void)
 	struct pvclock_vcpu_time_info *src;
 	cycle_t ret;
 
-	src = &get_cpu_var(hv_clock);
+	preempt_disable_notrace();
+	src = &__get_cpu_var(hv_clock);
 	ret = pvclock_clocksource_read(src);
-	put_cpu_var(hv_clock);
+	preempt_enable_notrace();
 	return ret;
 }
 


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