serial 8250: sysrq deadlock fix

Fix http://bugzilla.kernel.org/show_bug.cgi?id=6716

Doing a sysrq over a serial line into an SMP machine presently deadlocks.

Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Enrico Scholz <enrico.scholz@informatik.tu-chemnitz.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/serial/8250.c

@@ -2241,10 +2241,14 @@ serial8250_console_write(struct console *co, const char *s, unsigned int count)
 
 	touch_nmi_watchdog();
 
-	if (oops_in_progress) {
-		locked = spin_trylock_irqsave(&up->port.lock, flags);
+	local_irq_save(flags);
+	if (up->port.sysrq) {
+		/* serial8250_handle_port() already took the lock */
+		locked = 0;
+	} else if (oops_in_progress) {
+		locked = spin_trylock(&up->port.lock);
 	} else
-		spin_lock_irqsave(&up->port.lock, flags);
+		spin_lock(&up->port.lock);
 
 	/*
 	 *	First save the IER then disable the interrupts
@@ -2266,7 +2270,8 @@ serial8250_console_write(struct console *co, const char *s, unsigned int count)
 	serial_out(up, UART_IER, ier);
 
 	if (locked)
-		spin_unlock_irqrestore(&up->port.lock, flags);
+		spin_unlock(&up->port.lock);
+	local_irq_restore(flags);
 }
 
 static int serial8250_console_setup(struct console *co, char *options)