[RFCv4 2/6] ext4: mballoc: Use ext4_lock_group() around calculations involving bb_free

[Ritesh Harjani]

Currently while doing block allocation grp->bb_free may be getting
modified if discard is happening in parallel.
For e.g. consider a case where there are lot of threads who have
preallocated lot of blocks and there is a thread which is trying
to discard all of this group's PA. Now it could happen that
we see all of those group's bb_free is zero and fail the allocation
while there is sufficient space if we free up all the PA.

So this patch takes the ext4_lock_group() around calculations involving
grp->bb_free in ext4_mb_good_group() & ext4_mb_good_group_nolock()

Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
---
 fs/ext4/mballoc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index da11a4a738bd..dcd05ff7c012 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2138,9 +2138,11 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,
 				     ext4_group_t group, int cr)
 {
 	struct ext4_group_info *grp = ext4_get_group_info(ac->ac_sb, group);
+	struct super_block *sb = ac->ac_sb;
 	ext4_grpblk_t free;
 	int ret = 0;
 
+	ext4_lock_group(sb, group);
 	free = grp->bb_free;
 	if (free == 0)
 		goto out;
@@ -2148,6 +2150,7 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,
 		goto out;
 	if (unlikely(EXT4_MB_GRP_BBITMAP_CORRUPT(grp)))
 		goto out;
+	ext4_unlock_group(sb, group);
 
 	/* We only do this if the grp has never been initialized */
 	if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
@@ -2156,8 +2159,10 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,
 			return ret;
 	}
 
+	ext4_lock_group(sb, group);
 	ret = ext4_mb_good_group(ac, group, cr);
 out:
+	ext4_unlock_group(sb, group);
 	return ret;
 }
 
-- 
2.21.0