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>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 484c9d1..d23056d 100644 (file)
--- 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 68b0351..3cfc343 100644 (file)
--- 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);