Merge tag 'pr-20141223-x86-vdso' of git://git.kernel.org/pub/scm/linux/kernel/git/luto/linux into x86/urgent
Pull VDSO fix from Andy Lutomirski:
"This is hopefully the last vdso fix for 3.19. It should be very
safe (it just adds a volatile).
I don't think it fixes an actual bug (the __getcpu calls in the
pvclock code may not have been needed in the first place), but
discussion on that point is ongoing.
It also fixes a big performance issue in 3.18 and earlier in which
the lsl instructions in vclock_gettime got hoisted so far up the
function that they happened even when the function they were in was
never called. n 3.19, the performance issue seems to be gone due to
the whims of my compiler and some interaction with a branch that's
now gone.
I'll hopefully have a much bigger overhaul of the pvclock code
for 3.20, but it needs careful review."
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index e7e9682..f556c48 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -80,9 +80,11 @@
/*
* Load per CPU data from GDT. LSL is faster than RDTSCP and
- * works on all CPUs.
+ * works on all CPUs. This is volatile so that it orders
+ * correctly wrt barrier() and to keep gcc from cleverly
+ * hoisting it out of the calling function.
*/
- asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
+ asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
return p;
}