[RFC PATCH v1 3/6] exfat: use atomic bit ops for volume dirty flag

[Chi Zhiling]

From: Chi Zhiling <chizhiling@xxxxxxxxxx>

After converting s_lock to a reader-writer lock, multiple operations can
execute concurrently under the read lock. As a result, multiple threads may
concurrently update sbi->vol_flags, causing a race in the existing
read-modify-write sequence.

Convert sbi->vol_flags to an unsigned long bitmap and use
test_and_set_bit() / test_and_clear_bit() to atomically update the volume
dirty bit. Only the thread performing the 0 -> 1 transition updates the
boot sector, avoiding concurrent unsynchronized modifications of vol_flags
and the boot-sector buffer.

Remove vol_flags_persistent, since the bitmap can directly retain the
MEDIA_FAILURE state while preserving the complete 16-bit on-disk volume
flags field when writing it back.

Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
---
 fs/exfat/exfat_fs.h  |  3 +--
 fs/exfat/exfat_raw.h |  4 ++--
 fs/exfat/super.c     | 28 ++++++++++++----------------
 3 files changed, 15 insertions(+), 20 deletions(-)

diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index a9131fe03302..f1505c013248 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -232,8 +232,7 @@ struct exfat_sb_info {
 	unsigned int num_FAT_sectors; /* num of FAT sectors */
 	unsigned int root_dir; /* root dir cluster */
 	unsigned int dentries_per_clu; /* num of dentries per cluster */
-	unsigned int vol_flags; /* volume flags */
-	unsigned int vol_flags_persistent; /* volume flags to retain */
+	unsigned long vol_flags; /* volume flags (bitmap) */
 	struct buffer_head *boot_bh; /* buffer_head of BOOT sector */
 
 	unsigned int map_clu; /* allocation bitmap start cluster */
diff --git a/fs/exfat/exfat_raw.h b/fs/exfat/exfat_raw.h
index ec70cd35bba0..222fee5c2adf 100644
--- a/fs/exfat/exfat_raw.h
+++ b/fs/exfat/exfat_raw.h
@@ -14,8 +14,8 @@
 
 #define EXFAT_MAX_FILE_LEN	255
 
-#define VOLUME_DIRTY		0x0002
-#define MEDIA_FAILURE		0x0004
+#define VOLUME_DIRTY_BIT	1
+#define MEDIA_FAILURE_BIT	2
 
 #define EXFAT_EOF_CLUSTER	0xFFFFFFFFu
 #define EXFAT_BAD_CLUSTER	0xFFFFFFF7u
diff --git a/fs/exfat/super.c b/fs/exfat/super.c
index a9ea36ba2693..491273d8eeb6 100644
--- a/fs/exfat/super.c
+++ b/fs/exfat/super.c
@@ -69,27 +69,18 @@ static int exfat_statfs(struct dentry *dentry, struct kstatfs *buf)
 	return 0;
 }
 
-static int exfat_set_vol_flags(struct super_block *sb, unsigned short new_flags)
+static int exfat_sync_vol_flags(struct super_block *sb)
 {
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
 	struct boot_sector *p_boot = (struct boot_sector *)sbi->boot_bh->b_data;
 
-	/* retain persistent-flags */
-	new_flags |= sbi->vol_flags_persistent;
-
-	/* flags are not changed */
-	if (sbi->vol_flags == new_flags)
-		return 0;
-
-	sbi->vol_flags = new_flags;
-
 	/* skip updating volume dirty flag,
 	 * if this volume has been mounted with read-only
 	 */
 	if (sb_rdonly(sb))
 		return 0;
 
-	p_boot->vol_flags = cpu_to_le16(new_flags);
+	p_boot->vol_flags = cpu_to_le16((unsigned short)READ_ONCE(sbi->vol_flags));
 
 	set_buffer_uptodate(sbi->boot_bh);
 	mark_buffer_dirty(sbi->boot_bh);
@@ -103,14 +94,20 @@ int exfat_set_volume_dirty(struct super_block *sb)
 {
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
 
-	return exfat_set_vol_flags(sb, sbi->vol_flags | VOLUME_DIRTY);
+	if (test_and_set_bit(VOLUME_DIRTY_BIT, &sbi->vol_flags))
+		return 0;
+
+	return exfat_sync_vol_flags(sb);
 }
 
 int exfat_clear_volume_dirty(struct super_block *sb)
 {
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
 
-	return exfat_set_vol_flags(sb, sbi->vol_flags & ~VOLUME_DIRTY);
+	if (!test_and_clear_bit(VOLUME_DIRTY_BIT, &sbi->vol_flags))
+		return 0;
+
+	return exfat_sync_vol_flags(sb);
 }
 
 static int exfat_show_options(struct seq_file *m, struct dentry *root)
@@ -509,7 +506,6 @@ static int exfat_read_boot_sector(struct super_block *sb)
 		(sbi->cluster_size_bits - DENTRY_SIZE_BITS);
 
 	sbi->vol_flags = le16_to_cpu(p_boot->vol_flags);
-	sbi->vol_flags_persistent = sbi->vol_flags & (VOLUME_DIRTY | MEDIA_FAILURE);
 	sbi->clu_srch_ptr = EXFAT_FIRST_CLUSTER;
 
 	/* check consistencies */
@@ -526,9 +522,9 @@ static int exfat_read_boot_sector(struct super_block *sb)
 		return -EINVAL;
 	}
 
-	if (sbi->vol_flags & VOLUME_DIRTY)
+	if (test_bit(VOLUME_DIRTY_BIT, &sbi->vol_flags))
 		exfat_warn(sb, "Volume was not properly unmounted. Some data may be corrupt. Please run fsck.");
-	if (sbi->vol_flags & MEDIA_FAILURE)
+	if (test_bit(MEDIA_FAILURE_BIT, &sbi->vol_flags))
 		exfat_warn(sb, "Medium has reported failures. Some data may be lost.");
 
 	/*
-- 
2.53.0