[RFC PATCH v1 2/6] exfat: take bitmap_lock at the start of exfat_alloc_cluster()

[Chi Zhiling]

From: Chi Zhiling <chizhiling@xxxxxxxxxx>

Acquire sbi->bitmap_lock at the top of exfat_alloc_cluster() so the
used_clusters/clu_srch_ptr validation and updates are covered by the
lock, removing a window where the early checks raced with concurrent
allocations.

Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
---
 fs/exfat/fatent.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/fs/exfat/fatent.c b/fs/exfat/fatent.c
index a8b11e2ce43f..2db959669563 100644
--- a/fs/exfat/fatent.c
+++ b/fs/exfat/fatent.c
@@ -427,19 +427,22 @@ int exfat_alloc_cluster(struct inode *inode, unsigned int num_alloc,
 	struct super_block *sb = inode->i_sb;
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
 
+	mutex_lock(&sbi->bitmap_lock);
+
 	total_cnt = EXFAT_DATA_CLUSTER_COUNT(sbi);
 
 	if (unlikely(total_cnt < sbi->used_clusters)) {
 		exfat_fs_error_ratelimit(sb,
 			"%s: invalid used clusters(t:%u,u:%u)\n",
 			__func__, total_cnt, sbi->used_clusters);
-		return -EIO;
+		ret = -EIO;
+		goto unlock;
 	}
 
-	if (num_alloc > total_cnt - sbi->used_clusters)
-		return -ENOSPC;
-
-	mutex_lock(&sbi->bitmap_lock);
+	if (num_alloc > total_cnt - sbi->used_clusters) {
+		ret = -ENOSPC;
+		goto unlock;
+	}
 
 	hint_clu = p_chain->dir;
 	/* find new cluster */
@@ -516,8 +519,8 @@ int exfat_alloc_cluster(struct inode *inode, unsigned int num_alloc,
 done:
 			sbi->clu_srch_ptr = hint_clu;
 			sbi->used_clusters += p_chain->size;
-			mutex_unlock(&sbi->bitmap_lock);
-			return 0;
+			ret = 0;
+			goto unlock;
 		}
 
 		hint_clu = new_clu + 1;
-- 
2.53.0