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>

2 files changed, 6 insertions, 8 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)) {
diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index 68b0351fc647..3cfc343c41b5 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -746,7 +746,6 @@ int ext4_group_add(struct super_block *sb, struct ext4_new_group_data *input)
         struct inode *inode = NULL;
         handle_t *handle;
         int gdb_off, gdb_num;
-        int num_grp_locked = 0;
         int err, err2;
 
         gdb_num = input->group / EXT4_DESC_PER_BLOCK(sb);
@@ -856,7 +855,6 @@ int ext4_group_add(struct super_block *sb, struct ext4_new_group_data *input)
          * using the new disk blocks.
          */
 
-        num_grp_locked = ext4_mb_get_buddy_cache_lock(sb, input->group);
         /* Update group descriptor block for new group */
         gdp = (struct ext4_group_desc *)((char *)primary->b_data +
                                          gdb_off * EXT4_DESC_SIZE(sb));
@@ -875,10 +873,8 @@ int ext4_group_add(struct super_block *sb, struct ext4_new_group_data *input)
          * descriptor
          */
         err = ext4_mb_add_groupinfo(sb, input->group, gdp);
-        if (err) {
-                ext4_mb_put_buddy_cache_lock(sb, input->group, num_grp_locked);
+        if (err)
                 goto exit_journal;
-        }
 
         /*
          * Make the new blocks and inodes valid next.  We do this before
@@ -920,7 +916,6 @@ int ext4_group_add(struct super_block *sb, struct ext4_new_group_data *input)
 
         /* Update the global fs size fields */
         sbi->s_groups_count++;
-        ext4_mb_put_buddy_cache_lock(sb, input->group, num_grp_locked);
 
         ext4_handle_dirty_metadata(handle, NULL, primary);