[PATCH 04/08] dm: use spinlock for _minor_lock

[Jeff Mahoney]

 In order to truly serialize reference counting and properly handle the final
 dm_put, we need to use atomic_dec_and_lock, since another thread of execution
 can attempt to "resurrect" a potentially dead mapped_device.

 This patch replaces the _minor_lock mutex with a spinlock, and leverages
 atomic_dec_and_lock to properly serialize reference counting.

Signed-off-by: Jeff Mahoney <jeffm@xxxxxxxx>
--
 drivers/md/dm.c |   27 +++++++++++++--------------
 1 files changed, 13 insertions(+), 14 deletions(-)

diff -ruNpX ../dontdiff linux-2.6.16-staging1/drivers/md/dm.c linux-2.6.16-staging2/drivers/md/dm.c
--- linux-2.6.16-staging1/drivers/md/dm.c	2006-04-13 16:18:19.000000000 -0400
+++ linux-2.6.16-staging2/drivers/md/dm.c	2006-04-13 16:18:19.000000000 -0400
@@ -23,6 +23,7 @@ static const char *_name = DM_NAME;
 static unsigned int major = 0;
 static unsigned int _major = 0;
 
+static DEFINE_SPINLOCK(_minor_lock);
 /*
  * One of these is allocated per bio.
  */
@@ -697,14 +698,13 @@ static int dm_any_congested(void *conges
 /*-----------------------------------------------------------------
  * An IDR is used to keep track of allocated minor numbers.
  *---------------------------------------------------------------*/
-static DECLARE_MUTEX(_minor_lock);
 static DEFINE_IDR(_minor_idr);
 
 static void free_minor(unsigned int minor)
 {
-	down(&_minor_lock);
+	spin_lock(&_minor_lock);
 	idr_remove(&_minor_idr, minor);
-	up(&_minor_lock);
+	spin_unlock(&_minor_lock);
 }
 
 /*
@@ -721,7 +721,7 @@ static int specific_minor(struct mapped_
 	if (!r)
 		return -ENOMEM;
 
-	down(&_minor_lock);
+	spin_lock(&_minor_lock);
 
 	if (idr_find(&_minor_idr, minor)) {
 		r = -EBUSY;
@@ -740,7 +740,7 @@ static int specific_minor(struct mapped_
 	}
 
 out:
-	up(&_minor_lock);
+	spin_unlock(&_minor_lock);
 	return r;
 }
 
@@ -753,7 +753,7 @@ static int next_free_minor(struct mapped
 	if (!r)
 		return -ENOMEM;
 
-	down(&_minor_lock);
+	spin_lock(&_minor_lock);
 
 	r = idr_get_new(&_minor_idr, MINOR_ALLOCED, &m);
 	if (r) {
@@ -769,7 +769,7 @@ static int next_free_minor(struct mapped
 	*minor = m;
 
 out:
-	up(&_minor_lock);
+	spin_unlock(&_minor_lock);
 	return r;
 }
 
@@ -839,9 +839,9 @@ static struct mapped_device *alloc_dev(u
 	init_waitqueue_head(&md->eventq);
 
 	/* Populate the mapping, nobody knows we exist yet */
-	down(&_minor_lock);
+	spin_lock(&_minor_lock);
 	r = idr_replace(&_minor_idr, md, minor);
-	up(&_minor_lock);
+	spin_unlock(&_minor_lock);
 	BUG_ON(r < 0);
 
 	return md;
@@ -964,13 +964,13 @@ static struct mapped_device *dm_find_md(
 	if (MAJOR(dev) != _major || minor >= (1 << MINORBITS))
 		return NULL;
 
-	down(&_minor_lock);
+	spin_lock(&_minor_lock);
 
 	md = idr_find(&_minor_idr, minor);
 	if (md && (md == MINOR_ALLOCED || (dm_disk(md)->first_minor != minor)))
 		md = NULL;
 
-	up(&_minor_lock);
+	spin_unlock(&_minor_lock);
 
 	return md;
 }
@@ -1010,10 +1010,9 @@ void dm_put(struct mapped_device *md)
 {
 	struct dm_table *map = dm_get_table(md);
 
-	if (atomic_dec_and_test(&md->holders)) {
-		down(&_minor_lock);
+	if (atomic_dec_and_lock(&md->holders, &_minor_lock)) {
 		idr_replace(&_minor_idr, MINOR_ALLOCED, md->disk->first_minor);
-		up(&_minor_lock);
+		spin_unlock(&_minor_lock);
 		if (!dm_suspended(md)) {
 			dm_table_presuspend_targets(map);
 			dm_table_postsuspend_targets(map);
-
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