Handle page fault atomicity correctly when preempt is enabled.

arch/i386/mm/fault.c

@@ -184,7 +184,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long error_code)
 	 * If we're in an interrupt, have no user context or are running in an
 	 * atomic region then we must not take the fault..
 	 */
-	if (preempt_count() || !mm)
+	if (in_atomic() || !mm)
 		goto no_context;
 
 	down_read(&mm->mmap_sem);

include/asm-i386/hardirq.h

@@ -77,8 +77,10 @@ typedef struct {
 #define irq_enter()		(preempt_count() += HARDIRQ_OFFSET)
 
 #if CONFIG_PREEMPT
+# define in_atomic()	(preempt_count() != kernel_locked())
 # define IRQ_EXIT_OFFSET (HARDIRQ_OFFSET-1)
 #else
+# define in_atomic()	(preempt_count() != 0)
 # define IRQ_EXIT_OFFSET HARDIRQ_OFFSET
 #endif
 #define irq_exit()							\