[GIT PULL] Little bugfix to prevent recursion with tracing timestamps

[Jeremy Fitzhardinge]

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Hi Linus,

When starting a PV kernel under Xen with full boot-time trace testing
enabled, it crashes due to infinite recursion.  In principle it could
happen at other times too (when just using preempt tracing perhaps?). 
This fixes it.

BTW, I think this same problem also affects KVM.

Erm, I'm not sure how to authenticate this github as being really from
me; I'm signing the mail with my long-standing pgp key in the hope that
it helps...  Also, the patch is self-evident.

Thanks,
    J

The following changes since commit b6fd41e29dea9c6753b1843a77e50433e6123bcb:

  Linux 3.1-rc6 (2011-09-12 14:02:02 -0700)

are available in the git repository at:
  git://github.com/jsgf/linux-xen.git upstream/bugfix

Jeremy Fitzhardinge (1):
      xen: use non-tracing preempt in xen_clocksource_read()

 arch/x86/xen/time.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 5158c50..163b467 100644
- --- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -168,9 +168,10 @@ cycle_t xen_clocksource_read(void)
         struct pvclock_vcpu_time_info *src;
     cycle_t ret;
 
- -    src = &get_cpu_var(xen_vcpu)->time;
+    preempt_disable_notrace();
+    src = &__get_cpu_var(xen_vcpu)->time;
     ret = pvclock_clocksource_read(src);
- -    put_cpu_var(xen_vcpu);
+    preempt_enable_notrace();
     return ret;
 }
 
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