fix lockdep warnings

Hello,

There is a lockdep warning that pops up complaining about grabbing the block_group->alloc_mutex while holding the fs_info->pinned_mutex. This is because in cache_block_group we grabe the pinned_mutex while holding the alloc_mutex. This patch fixes this particular complaint by adding a cache_mutex that will be held when caching the block group and no other time. This will keep the lockdep warning from happening, and is a little cleaner. I also added a test to see if the block group is cached before calling cache_block_group in find_free_extent to keep us from checking the block group needlessly, since really you are only going to need to call cache_block_group once, and every time after that you will be fine. Thank you,

index abb2733..8d9c2b4 100644
@@ -561,6 +561,7 @@ struct btrfs_block_group_cache {
 	struct btrfs_block_group_item item;
 	spinlock_t lock;
 	struct mutex alloc_mutex;
+	struct mutex cache_mutex;
 	u64 pinned;
 	u64 reserved;
 	u64 flags;

index 155c8dc..59690f8 100644
@@ -158,8 +158,8 @@ static int add_new_free_space(struct btrfs_block_group_cache *block_group,
 			start = extent_end + 1;
 		} else if (extent_start > start && extent_start < end) {
 			size = extent_start - start;
-			ret = btrfs_add_free_space_lock(block_group, start,
-							size);
+			ret = btrfs_add_free_space(block_group, start,
+						   size);
 			BUG_ON(ret);
 			start = extent_end + 1;
 		} else {

@@ -169,7 +169,7 @@ static int add_new_free_space(struct btrfs_block_group_cache *block_group,
 
 	if (start < end) {
 		size = end - start;
-		ret = btrfs_add_free_space_lock(block_group, start, size);
+		ret = btrfs_add_free_space(block_group, start, size);
 		BUG_ON(ret);
 	}
 	mutex_unlock(&info->pinned_mutex);

@@ -2247,17 +2247,20 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 		 * should never happen
 		 */
 		WARN_ON(!block_group);
+
+		if (unlikely(!block_group->cached)) {
+			mutex_lock(&block_group->cache_mutex);
+			ret = cache_block_group(root, block_group);
+			mutex_unlock(&block_group->cache_mutex);
+			if (ret)
+				break;
+		}
+
 		mutex_lock(&block_group->alloc_mutex);
 		if (unlikely(!block_group_bits(block_group, data)))
 			goto new_group;
 
-		ret = cache_block_group(root, block_group);
-		if (ret) {
-			mutex_unlock(&block_group->alloc_mutex);
-			break;
-		}
-
-		if (block_group->ro)
+		if (unlikely(block_group->ro))
 			goto new_group;
 
 		free_space = btrfs_find_free_space(block_group, search_start,

@@ -2630,12 +2633,12 @@ int btrfs_alloc_logged_extent(struct btrfs_trans_handle *trans,
 	struct btrfs_block_group_cache *block_group;
 
 	block_group = btrfs_lookup_block_group(root->fs_info, ins->objectid);
-	mutex_lock(&block_group->alloc_mutex);
+	mutex_lock(&block_group->cache_mutex);
 	cache_block_group(root, block_group);
+	mutex_unlock(&block_group->cache_mutex);
 
-	ret = btrfs_remove_free_space_lock(block_group, ins->objectid,
-					   ins->offset);
-	mutex_unlock(&block_group->alloc_mutex);
+	ret = btrfs_remove_free_space(block_group, ins->objectid,
+				      ins->offset);
 	BUG_ON(ret);
 	ret = __btrfs_alloc_reserved_extent(trans, root, parent, root_objectid,
 					    ref_generation, owner, ins);

@@ -5156,6 +5159,7 @@ int btrfs_read_block_groups(struct btrfs_root *root)
 
 		spin_lock_init(&cache->lock);
 		mutex_init(&cache->alloc_mutex);
+		mutex_init(&cache->cache_mutex);
 		INIT_LIST_HEAD(&cache->list);
 		read_extent_buffer(leaf, &cache->item,
 				   btrfs_item_ptr_offset(leaf, path->slots[0]),

@@ -5207,6 +5211,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans,
 	cache->key.offset = size;
 	spin_lock_init(&cache->lock);
 	mutex_init(&cache->alloc_mutex);
+	mutex_init(&cache->cache_mutex);
 	INIT_LIST_HEAD(&cache->list);
 	btrfs_set_key_type(&cache->key, BTRFS_BLOCK_GROUP_ITEM_KEY);
 

Josef,

I saw this yesterday, but not sure how to reproduce it. Is this the same warning that this patch fixes ?

---------------------------------

Btrfs loaded
device fsid 51464cf7df326b76-8d5a548c72c2aaa9 devid 1 transid 13 /dev/sdb1
SELinux: initialized (dev sdb1, type btrfs), not configured for labeling

=======================================================
[ INFO: possible circular locking dependency detected ] 2.6.27 #2
-------------------------------------------------------
btrfs-transacti/2651 is trying to acquire lock:
(&cache->alloc_mutex){--..}, at: [<e0b92387>]
btrfs_add_free_space+0x1c/0x65 [btrfs]

but task is already holding lock:
(&fs_info->pinned_mutex){--..}, at: [<e0b662fe>]
btrfs_finish_extent_commit+0x20/0xf9 [btrfs]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&fs_info->pinned_mutex){--..}:

       [<c0447cbb>] validate_chain+0x7fe/0xa43
       [<c0448574>] __lock_acquire+0x674/0x6da
       [<c0448635>] lock_acquire+0x5b/0x81
       [<c064c039>] mutex_lock_nested+0xd1/0x259
       [<e0b62aa6>] add_new_free_space+0x27/0xd3 [btrfs]
       [<e0b62d71>] cache_block_group+0x21f/0x29d [btrfs]
       [<e0b64d25>] find_free_extent+0x239/0x519 [btrfs]
       [<e0b651a1>] __btrfs_reserve_extent+0x19c/0x33b [btrfs]
       [<e0b656c6>] btrfs_alloc_extent+0x40/0xa5 [btrfs]
       [<e0b6577b>] btrfs_alloc_free_block+0x50/0x7a [btrfs]
       [<e0b5d925>] __btrfs_cow_block+0x1bb/0x65d [btrfs]
       [<e0b5e330>] btrfs_cow_block+0x19d/0x1a8 [btrfs]
       [<e0b6099f>] btrfs_search_slot+0x281/0x627 [btrfs]
       [<e0b6c8dd>] btrfs_lookup_inode+0x28/0x86 [btrfs]
       [<e0b7408f>] btrfs_update_inode+0x3d/0xa4 [btrfs]
       [<e0b7488b>] btrfs_dirty_inode+0x3c/0x4b [btrfs]
       [<c04a844f>] __mark_inode_dirty+0x26/0x13f
       [<c04a007a>] touch_atime+0xb6/0xbc
       [<c04992c6>] vfs_readdir+0x7b/0x94
       [<c049933d>] sys_getdents64+0x5e/0xa0
       [<c0403973>] sysenter_do_call+0x12/0x3f
       [<ffffffff>] 0xffffffff

-> #0 (&cache->alloc_mutex){--..}:

       [<c0447a44>] validate_chain+0x587/0xa43
       [<c0448574>] __lock_acquire+0x674/0x6da
       [<c0448635>] lock_acquire+0x5b/0x81
       [<c064c039>] mutex_lock_nested+0xd1/0x259
       [<e0b92387>] btrfs_add_free_space+0x1c/0x65 [btrfs]
       [<e0b66390>] btrfs_finish_extent_commit+0xb2/0xf9 [btrfs]
       [<e0b71d01>] btrfs_commit_transaction+0x512/0x65d [btrfs]
       [<e0b6e8d2>] transaction_kthread+0x164/0x1dc [btrfs]
       [<c043a8f8>] kthread+0x3b/0x63
       [<c0404717>] kernel_thread_helper+0x7/0x10
       [<ffffffff>] 0xffffffff

other info that might help us debug this:

3 locks held by btrfs-transacti/2651:
#0: (&fs_info->transaction_kthread_mutex){--..}, at: [<e0b6e81e>]

transaction_kthread+0xb0/0x1dc [btrfs]
 #1:  (&fs_info->tree_reloc_mutex){--..}, at: [<e0b71a94>]
btrfs_commit_transaction+0x2a5/0x65d [btrfs]
 #2:  (&fs_info->pinned_mutex){--..}, at: [<e0b662fe>]
btrfs_finish_extent_commit+0x20/0xf9 [btrfs]

stack backtrace:
Pid: 2651, comm: btrfs-transacti Not tainted 2.6.27 #2
 [<c04474b2>] print_circular_bug_tail+0x9b/0xa6
 [<c0447a44>] validate_chain+0x587/0xa43
 [<c0444caa>] ? trace_hardirqs_off+0xb/0xd
 [<c0448574>] __lock_acquire+0x674/0x6da
 [<c0448635>] lock_acquire+0x5b/0x81
 [<e0b92387>] ? btrfs_add_free_space+0x1c/0x65 [btrfs]
 [<c064c039>] mutex_lock_nested+0xd1/0x259
 [<e0b92387>] ? btrfs_add_free_space+0x1c/0x65 [btrfs]
 [<e0b92387>] ? btrfs_add_free_space+0x1c/0x65 [btrfs]
 [<e0b92387>] btrfs_add_free_space+0x1c/0x65 [btrfs]
 [<e0b66390>] btrfs_finish_extent_commit+0xb2/0xf9 [btrfs]
 [<e0b71d01>] btrfs_commit_transaction+0x512/0x65d [btrfs]
 [<c043a9b9>] ? autoremove_wake_function+0x0/0x30
 [<e0b6e8d2>] transaction_kthread+0x164/0x1dc [btrfs]
 [<c041f6be>] ? complete+0x34/0x3e
 [<e0b6e76e>] ? transaction_kthread+0x0/0x1dc [btrfs]
 [<c043a8f8>] kthread+0x3b/0x63
 [<c043a8bd>] ? kthread+0x0/0x63
 [<c0404717>] kernel_thread_helper+0x7/0x10
 =======================

device fsid 51464cf7df326b76-8d5a548c72c2aaa9 devid 1 transid 20 /dev/sdb1

Yup thats the warning this patch is supposed to fix. You didn't get this warning with this patch did you? Thanks,

Josef