[PATCH] lockdep: avoid lockdep warning in md

md_open takes ->reconfig_mutex which causes lockdep to complain.  This
(normally) doesn't have deadlock potential as the possible conflict is with a
reconfig_mutex in a different device.

I say "normally" because if a loop were created in the array->member hierarchy
a deadlock could happen.  However that causes bigger problems than a deadlock
and should be fixed independently.

So we flag the lock in md_open as a nested lock.  This requires defining
mutex_lock_interruptible_nested.

Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/md/md.c

@@ -4423,7 +4423,7 @@ static int md_open(struct inode *inode, struct file *file)
 	mddev_t *mddev = inode->i_bdev->bd_disk->private_data;
 	int err;
 
-	if ((err = mddev_lock(mddev)))
+	if ((err = mutex_lock_interruptible_nested(&mddev->reconfig_mutex, 1)))
 		goto out;
 
 	err = 0;

include/linux/mutex.h

@@ -125,8 +125,10 @@ extern int fastcall mutex_lock_interruptible(struct mutex *lock);
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 extern void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
+extern int mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass);
 #else
 # define mutex_lock_nested(lock, subclass) mutex_lock(lock)
+# define mutex_lock_interruptible_nested(lock, subclass) mutex_lock_interruptible(lock)
 #endif
 
 /*

kernel/mutex.c

@@ -206,6 +206,15 @@ mutex_lock_nested(struct mutex *lock, unsigned int subclass)
 }
 
 EXPORT_SYMBOL_GPL(mutex_lock_nested);
+
+int __sched
+mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
+{
+	might_sleep();
+	return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, subclass);
+}
+
+EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
 #endif
 
 /*