ext4: lock block groups when initializing

I noticed when filling a 1T filesystem with 4 threads using the
fs_mark benchmark:

fs_mark -d /mnt/test -D 256 -n 100000 -t 4 -s 20480 -F -S 0

that I occasionally got checksum mismatch errors:

EXT4-fs error (device sdb): ext4_init_inode_bitmap: Checksum bad for group 6935

etc.  I'd reliably get 4-5 of them during the run.

It appears that the problem is likely a race to init the bg's
when the uninit_bg feature is enabled.

With the patch below, which adds sb_bgl_locking around initialization,
I was able to complete several runs with no errors or warnings.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 386cb79ac4b6..1ae5004e93fc 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -321,12 +321,15 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
         if (bh_uptodate_or_lock(bh))
                 return bh;
 
+        spin_lock(sb_bgl_lock(EXT4_SB(sb), block_group));
         if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
                 ext4_init_block_bitmap(sb, bh, block_group, desc);
                 set_buffer_uptodate(bh);
                 unlock_buffer(bh);
+                spin_unlock(sb_bgl_lock(EXT4_SB(sb), block_group));
                 return bh;
         }
+        spin_unlock(sb_bgl_lock(EXT4_SB(sb), block_group));
         if (bh_submit_read(bh) < 0) {
                 put_bh(bh);
                 ext4_error(sb, __func__,