MTRR: Fix race causing set_mtrr to go into infinite loop

Processors synchronization in set_mtrr requires the .gate field to be set
after .count field is properly initialized.  Without an explicit barrier,
the compiler was reordering those memory stores.  That was sometimes
causing a processor (in ipi_handler) to see the .gate change and decrement
.count before the latter is set by set_mtrr() (which then hangs in a
infinite loop with irqs disabled).
Signed-off-by: NLoic Prylli <loic@myri.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/i386/kernel/cpu/mtrr/main.c

@@ -229,6 +229,8 @@ static void set_mtrr(unsigned int reg, unsigned long base,
 	data.smp_size = size;
 	data.smp_type = type;
 	atomic_set(&data.count, num_booting_cpus() - 1);
+	/* make sure data.count is visible before unleashing other CPUs */
+	smp_wmb();
 	atomic_set(&data.gate,0);
 
 	/*  Start the ball rolling on other CPUs  */
@@ -242,6 +244,7 @@ static void set_mtrr(unsigned int reg, unsigned long base,
 
 	/* ok, reset count and toggle gate */
 	atomic_set(&data.count, num_booting_cpus() - 1);
+	smp_wmb();
 	atomic_set(&data.gate,1);
 
 	/* do our MTRR business */
@@ -260,6 +263,7 @@ static void set_mtrr(unsigned int reg, unsigned long base,
 		cpu_relax();
 
 	atomic_set(&data.count, num_booting_cpus() - 1);
+	smp_wmb();
 	atomic_set(&data.gate,0);
 
 	/*