[PATCH] utilization of kprobe_mutex is incorrect on x86_64

The up()/down() orders are incorrect in arch/x86_64/kprobes.c file.
kprobe_mutext is used to protect the free kprobe instruction slot list.
arch_prepare_kprobe applies for a slot from the free list, and
arch_remove_kprobe returns a slot to the free list.  The incorrect up()/down()
orders to operate on kprobe_mutex fail to protect the free list.  If 2 threads
try to get/return kprobe instruction slot at the same time, the free slot list
might be broken, or a free slot might be applied by 2 threads.

Signed-off-by: Zhang Yanmin <Yanmin.zhang@intel.com>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/kprobes.c

@@ -77,9 +77,9 @@ static inline int is_IF_modifier(kprobe_opcode_t *insn)
 int __kprobes arch_prepare_kprobe(struct kprobe *p)
 {
 	/* insn: must be on special executable page on x86_64. */
-	up(&kprobe_mutex);
-	p->ainsn.insn = get_insn_slot();
 	down(&kprobe_mutex);
+	p->ainsn.insn = get_insn_slot();
+	up(&kprobe_mutex);
 	if (!p->ainsn.insn) {
 		return -ENOMEM;
 	}
@@ -231,9 +231,9 @@ void __kprobes arch_disarm_kprobe(struct kprobe *p)
 
 void __kprobes arch_remove_kprobe(struct kprobe *p)
 {
-	up(&kprobe_mutex);
-	free_insn_slot(p->ainsn.insn);
 	down(&kprobe_mutex);
+	free_insn_slot(p->ainsn.insn);
+	up(&kprobe_mutex);
 }
 
 static inline void save_previous_kprobe(void)