Re: md deadlock (was Re: 2.6.18-mm2)

Neil Brown
2006-10-10T03:53:28Z

Hi,
would this be an appropriate fix do the warning lockdep gives about
possible deadlocks in md.

The warning is currently easily triggered with
mdadm -C /dev/md1 -l1 -n1 /dev/sdc missing

(assuming /dev/sdc is a device that you are happy to be scribbled on).

This will take ->reconfig_mutex on md1 while holding bd_mutex,
then will take bd_mutex on sdc while holding reconfig_mutex on md1

This superficial deadlock isn't a real problem because the bd_mutexes
are on different devices and there is an hierarchical relationship
which avoids the loop necessary for a deadlock.

-----------------------
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.

Signed-off-by: Neil Brown <neilb@suse.de>

### Diffstat output
./drivers/md/md.c | 2 +-
./include/linux/mutex.h | 3 ++-
./kernel/mutex.c | 8 ++++++++
3 files changed, 11 insertions(+), 2 deletions(-)

diff .prev/drivers/md/md.c ./drivers/md/md.c
--- .prev/drivers/md/md.c 2006-10-09 14:25:11.000000000 +1000
+++ ./drivers/md/md.c 2006-10-10 12:28:35.000000000 +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 2006-10-10 12:37:04.000000000 +1000
+++ ./include/linux/mutex.h 2006-10-10 12:40:20.000000000 +1000
@@ -125,8 +125,9 @@ 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_interruptible_lock(lock)
#endif

/*

diff .prev/kernel/mutex.c ./kernel/mutex.c
--- .prev/kernel/mutex.c 2006-10-10 12:35:54.000000000 +1000
+++ ./kernel/mutex.c 2006-10-10 13:20:04.000000000 +1000
@@ -206,6 +206,14 @@ 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

/*