powerpc: introduce die_mce

As explained by commit daf00ae71d ("powerpc/traps: restore
recoverability of machine_check interrupts"), die() can't be called from
within nmi_enter to nicely kill a process context that was interrupted.
nmi_exit must be called first.

This adds a function die_mce which takes care of this for machine check
handlers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-24-npiggin@gmail.com

arch/powerpc/include/asm/bug.h

@@ -118,6 +118,7 @@ void do_bad_page_fault_segv(struct pt_regs *regs);
 extern void _exception(int, struct pt_regs *, int, unsigned long);
 extern void _exception_pkey(struct pt_regs *, unsigned long, int);
 extern void die(const char *, struct pt_regs *, long);
+void die_mce(const char *str, struct pt_regs *regs, long err);
 extern bool die_will_crash(void);
 extern void panic_flush_kmsg_start(void);
 extern void panic_flush_kmsg_end(void);

arch/powerpc/kernel/traps.c

@@ -789,6 +789,19 @@ int machine_check_generic(struct pt_regs *regs)
 }
 #endif /* everything else */
 
+void die_mce(const char *str, struct pt_regs *regs, long err)
+{
+	/*
+	 * The machine check wants to kill the interrupted context, but
+	 * do_exit() checks for in_interrupt() and panics in that case, so
+	 * exit the irq/nmi before calling die.
+	 */
+	if (!IS_ENABLED(CONFIG_PPC_BOOK3S_64))
+		nmi_exit();
+	die(str, regs, err);
+}
+NOKPROBE_SYMBOL(die_mce);
+
 void machine_check_exception(struct pt_regs *regs)
 {
 	int recover = 0;
@@ -831,15 +844,11 @@ void machine_check_exception(struct pt_regs *regs)
 	if (check_io_access(regs))
 		goto bail;
 
-	if (nmi) nmi_exit();
-
-	die("Machine check", regs, SIGBUS);
+	die_mce("Machine check", regs, SIGBUS);
 
 	/* Must die if the interrupt is not recoverable */
 	if (!(regs->msr & MSR_RI))
-		die("Unrecoverable Machine check", regs, SIGBUS);
-
-	return;
+		die_mce("Unrecoverable Machine check", regs, SIGBUS);
 
 bail:
 	if (nmi) nmi_exit();

arch/powerpc/platforms/powernv/opal.c

@@ -624,7 +624,7 @@ static int opal_recover_mce(struct pt_regs *regs,
 			 */
 			recovered = 0;
 		} else {
-			die("Machine check", regs, SIGBUS);
+			die_mce("Machine check", regs, SIGBUS);
 			recovered = 1;
 		}
 	}

arch/powerpc/platforms/pseries/ras.c

@@ -808,7 +808,7 @@ static int recover_mce(struct pt_regs *regs, struct machine_check_event *evt)
 			 */
 			recovered = 0;
 		} else {
-			die("Machine check", regs, SIGBUS);
+			die_mce("Machine check", regs, SIGBUS);
 			recovered = 1;
 		}
 	}