[PATCH v2] ext4: fix race in ext4_mb_check_group_pa

[rafad900 <rafad900@gmail.com>]

ext4_mb_check_group_pa drops the reference count on the previous
best PA using atomic_dec(&cpa->pa_count) without holding the
cpa->pa_lock.

This causes race with ext4_discard_preallocations() which checks
pa_count to decide whether a PA is still in use. If the pa_count
is dec between the check and the discard, the PA can be freed
while ext4_mb_check_group_pa() still holds a reference to
it.

Fix this by taking the cpa->pa_lock around the atomic_dec.
Similar to pa->pa_lock which is taken outside of the
ext4_mb_check_group_pa() function.

The race was found while testing a change related to a
Coccinelle warning from atomic_as_refcounter.cocci. The refcount
conversion was found to be incorrect but the change had revealed
the pre-exiting race condition.

Signed-off-by: rafad900 <rafad900@gmail.com>
---
Changes in v2:
- Identified correct race location: ext4_mb_check_group_pa rather
  than ext4_mb_use_preallocated (the inode PA path already holds
  pa_lock correctly)
- Dropped refcount_t conversion — incompatible with PA lifecycle
  where count=0 represents an idle but reusable PA
- Added spin_lock(&cpa->pa_lock) around atomic_dec in
  ext4_mb_check_group_pa

 fs/ext4/mballoc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index ed1bd00e11cd..c9a118ae4658 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4833,7 +4833,9 @@ ext4_mb_check_group_pa(ext4_fsblk_t goal_block,
 		return cpa;
 
 	/* drop the previous reference */
+	spin_lock(&cpa->pa_lock);
 	atomic_dec(&cpa->pa_count);
+	spin_unlock(&cpa->pa_lock);
 	atomic_inc(&pa->pa_count);
 	return pa;
 }
-- 
2.43.0