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/mballoc.c b/fs/ext4/mballoc.c
index 8d141a25bbee..4258d3289a6f 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -787,13 +787,16 @@ static int ext4_mb_init_cache(struct page *page, char *incore)
                 if (bh_uptodate_or_lock(bh[i]))
                         continue;
 
+                spin_lock(sb_bgl_lock(EXT4_SB(sb), first_group + i));
                 if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
                         ext4_init_block_bitmap(sb, bh[i],
                                                 first_group + i, desc);
                         set_buffer_uptodate(bh[i]);
                         unlock_buffer(bh[i]);
+                        spin_unlock(sb_bgl_lock(EXT4_SB(sb), first_group + i));
                         continue;
                 }
+                spin_unlock(sb_bgl_lock(EXT4_SB(sb), first_group + i));
                 get_bh(bh[i]);
                 bh[i]->b_end_io = end_buffer_read_sync;
                 submit_bh(READ, bh[i]);