x86, mce: use force_sig_info to kill process in machine check

Impact: bug fix (with tolerant == 3)

do_exit cannot be called directly from the exception handler because
it can sleep and the exception handler runs on the exception stack.
Use force_sig() instead.

Based on a earlier patch by Ying Huang who debugged the problem.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/cpu/mcheck/mce_64.c

@@ -295,11 +295,11 @@ void do_machine_check(struct pt_regs * regs, long error_code)
 		 * If we know that the error was in user space, send a
 		 * SIGBUS.  Otherwise, panic if tolerance is low.
 		 *
-		 * do_exit() takes an awful lot of locks and has a slight
+		 * force_sig() takes an awful lot of locks and has a slight
 		 * risk of deadlocking.
 		 */
 		if (user_space) {
-			do_exit(SIGBUS);
+			force_sig(SIGBUS, current);
 		} else if (panic_on_oops || tolerant < 2) {
 			mce_panic("Uncorrected machine check",
 				&panicm, mcestart);