powerpc: Fix unpaired probe_hcall_entry and probe_hcall_exit

Unpaired calling of probe_hcall_entry and probe_hcall_exit might happen
as following, which could cause incorrect preempt count.

__trace_hcall_entry => trace_hcall_entry -> probe_hcall_entry =>
get_cpu_var => preempt_disable

__trace_hcall_exit => trace_hcall_exit -> probe_hcall_exit =>
put_cpu_var => preempt_enable

where:
A => B and A -> B means A calls B, but
=> means A will call B through function name, and B will definitely be
called.
-> means A will call B through function pointer, so B might not be
called if the function pointer is not set.

So error happens when only one of probe_hcall_entry and probe_hcall_exit
get called during a hcall.

This patch tries to move the preempt count operations from
probe_hcall_entry and probe_hcall_exit to its callers.

Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: stable@kernel.org [v2.6.32+]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/hvCall_inst.c

@@ -109,7 +109,7 @@ static void probe_hcall_entry(void *ignored, unsigned long opcode, unsigned long
 	if (opcode > MAX_HCALL_OPCODE)
 		return;
 
-	h = &get_cpu_var(hcall_stats)[opcode / 4];
+	h = &__get_cpu_var(hcall_stats)[opcode / 4];
 	h->tb_start = mftb();
 	h->purr_start = mfspr(SPRN_PURR);
 }
@@ -126,8 +126,6 @@ static void probe_hcall_exit(void *ignored, unsigned long opcode, unsigned long
 	h->num_calls++;
 	h->tb_total += mftb() - h->tb_start;
 	h->purr_total += mfspr(SPRN_PURR) - h->purr_start;
-
-	put_cpu_var(hcall_stats);
 }
 
 static int __init hcall_inst_init(void)

arch/powerpc/platforms/pseries/lpar.c

@@ -554,6 +554,7 @@ void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
 		goto out;
 
 	(*depth)++;
+	preempt_disable();
 	trace_hcall_entry(opcode, args);
 	(*depth)--;
 
@@ -576,6 +577,7 @@ void __trace_hcall_exit(long opcode, unsigned long retval,
 
 	(*depth)++;
 	trace_hcall_exit(opcode, retval, retbuf);
+	preempt_enable();
 	(*depth)--;
 
 out: