[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.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 9 insertions, 4 deletions

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 5443fcd38355..0ae9ced00ed4 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -2252,10 +2252,14 @@ 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
@@ -2277,7 +2281,8 @@ 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)