ext4: Clarify the locking details in mballoc

We don't need to take the alloc_sem lock when we are adding new
groups, since mballoc won't see the new group added until we bump
sbi->s_groups_count.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 484c9d10c7fa..d23056d375b3 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -928,8 +928,11 @@ int ext4_mb_init_group(struct super_block *sb, ext4_group_t group)
         blocks_per_page = PAGE_CACHE_SIZE / sb->s_blocksize;
         this_grp = ext4_get_group_info(sb, group);
         /*
-         * This ensures we don't add group
-         * to this buddy cache via resize
+         * This ensures that we don't reinit the buddy cache
+         * page which map to the group from which we are already
+         * allocating. If we are looking at the buddy cache we would
+         * have taken a reference using ext4_mb_load_buddy and that
+         * would have taken the alloc_sem lock.
          */
         num_grp_locked =  ext4_mb_get_buddy_cache_lock(sb, group);
         if (!EXT4_MB_GRP_NEED_INIT(this_grp)) {