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, 2 insertions, 0 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 876e1c620365..511997ef6f0e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1621,6 +1621,7 @@ static int ext4_check_descriptors(struct super_block *sb)
                                "(block %llu)!", i, inode_table);
                         return 0;
                 }
+                spin_lock(sb_bgl_lock(sbi, i));
                 if (!ext4_group_desc_csum_verify(sbi, i, gdp)) {
                         printk(KERN_ERR "EXT4-fs: ext4_check_descriptors: "
                                "Checksum for group %lu failed (%u!=%u)\n",
@@ -1629,6 +1630,7 @@ static int ext4_check_descriptors(struct super_block *sb)
                         if (!(sb->s_flags & MS_RDONLY))
                                 return 0;
                 }
+                spin_unlock(sb_bgl_lock(sbi, i));
                 if (!flexbg_flag)
                         first_block += EXT4_BLOCKS_PER_GROUP(sb);
         }