enable interrupts in user path of page fault.

This is a minor fix, but what is currently there is essentially wrong.
In do_page_fault, if the faulting address from user code happens to be
in kernel address space (int *p = (int*)-1; p = 0xbed;)  then the
do_page_fault handler will jump over the local_irq_enable with the

  goto bad_area_nosemaphore;

But the first line there sees this is user code and goes through the
process of sending a signal to send SIGSEGV to the user task. This whole
time interrupts are disabled and the task can not be preempted by a
higher priority task.

This patch always enables interrupts in the user path of the
bad_area_nosemaphore.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/i386/mm/fault.c

@@ -458,6 +458,11 @@ bad_area:
 bad_area_nosemaphore:
 	/* User mode accesses just cause a SIGSEGV */
 	if (error_code & 4) {
+		/*
+		 * It's possible to have interrupts off here.
+		 */
+		local_irq_enable();
+
 		/* 
 		 * Valid to do another page fault here because this one came 
 		 * from user space.

arch/x86_64/mm/fault.c

@@ -476,6 +476,12 @@ bad_area:
 bad_area_nosemaphore:
 	/* User mode accesses just cause a SIGSEGV */
 	if (error_code & PF_USER) {
+
+		/*
+		 * It's possible to have interrupts off here.
+		 */
+		local_irq_enable();
+
 		if (is_prefetch(regs, address, error_code))
 			return;