[PATCH 004 of 4] Avoid lockdep warning in md.

[NeilBrown]

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@xxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Neil Brown <neilb@xxxxxxx>

### Diffstat output
 ./drivers/md/md.c       |    2 +-
 ./include/linux/mutex.h |    2 ++
 ./kernel/mutex.c        |    9 +++++++++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff .prev/drivers/md/md.c ./drivers/md/md.c
--- .prev/drivers/md/md.c	2006-10-11 15:53:48.000000000 +1000
+++ ./drivers/md/md.c	2006-10-11 15:42:09.000000000 +1000
@@ -4422,7 +4422,7 @@ static int md_open(struct inode *inode, 
 	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;

diff .prev/include/linux/mutex.h ./include/linux/mutex.h
--- .prev/include/linux/mutex.h	2006-10-11 15:53:48.000000000 +1000
+++ ./include/linux/mutex.h	2006-10-11 15:43:22.000000000 +1000
@@ -125,8 +125,10 @@ extern int fastcall mutex_lock_interrupt
 
 #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
 
 /*

diff .prev/kernel/mutex.c ./kernel/mutex.c
--- .prev/kernel/mutex.c	2006-10-11 15:53:48.000000000 +1000
+++ ./kernel/mutex.c	2006-10-11 15:45:08.000000000 +1000
@@ -206,6 +206,15 @@ mutex_lock_nested(struct mutex *lock, un
 }
 
 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
 
 /*
-
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