[PATCH] uml: console locking fixes

Clean up the console driver locking.  There are various problems here,
including sleeping under a spinlock and spinlock recursion, some of which are
fixed here.  This patch deals with the locking involved with opens and closes.
 The problem is that an mconsole request to change a console's configuration
can race with an open.  Changing a configuration should only be done when a
console isn't opened.  Also, an open must be looking at a stable
configuration.  In addition, a get configuration request must observe the same
locking since it must also see a stable configuration.  With the old locking,
it was possible for this to hang indefinitely in some cases because open would
block for a long time waiting for a connection from the host while holding the
lock needed by the mconsole request.

As explained in the long comment, this is fixed by adding a spinlock for the
use count and configuration and a mutex for the actual open and close.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 5 deletions

diff --git a/arch/um/include/line.h b/arch/um/include/line.h
index 5f232ae89fb..b79643eeee0 100644
--- a/arch/um/include/line.h
+++ b/arch/um/include/line.h
@@ -11,6 +11,7 @@
 #include "linux/tty.h"
 #include "linux/interrupt.h"
 #include "linux/spinlock.h"
+#include "linux/mutex.h"
 #include "chan_user.h"
 #include "mconsole_kern.h"
 
@@ -32,15 +33,17 @@ struct line_driver {
 
 struct line {
 	struct tty_struct *tty;
+	spinlock_t count_lock;
+	int valid;
+
+	struct mutex open_mutex;
 	char *init_str;
 	int init_pri;
 	struct list_head chan_list;
-	int valid;
-	int count;
-	int throttled;
+
 	/*This lock is actually, mostly, local to*/
 	spinlock_t lock;
-
+	int throttled;
 	/* Yes, this is a real circular buffer.
 	 * XXX: And this should become a struct kfifo!
 	 *
@@ -57,7 +60,8 @@ struct line {
 };
 
 #define LINE_INIT(str, d) \
-	{ .init_str =	str, \
+	{ .count_lock =	SPIN_LOCK_UNLOCKED, \
+	  .init_str =	str,	\
 	  .init_pri =	INIT_STATIC, \
 	  .valid =	1, \
 	  .lock =	SPIN_LOCK_UNLOCKED, \