fat: Fix the race of read/write the FAT12 entry

FAT12 entry is 12bits, so it needs 2 phase to update the value.  And
writer and reader access it without any lock, so reader can get the
half updated value.

This fixes the long standing race condition by adding a global
spinlock to only FAT12 for avoiding any impact against FAT16/32.
Signed-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/fat/fatent.c

@@ -17,6 +17,8 @@ struct fatent_operations {
 	int (*ent_next)(struct fat_entry *);
 };
 
+static DEFINE_SPINLOCK(fat12_entry_lock);
+
 static void fat12_ent_blocknr(struct super_block *sb, int entry,
 			      int *offset, sector_t *blocknr)
 {
@@ -116,10 +118,13 @@ static int fat12_ent_get(struct fat_entry *fatent)
 	u8 **ent12_p = fatent->u.ent12_p;
 	int next;
 
+	spin_lock(&fat12_entry_lock);
 	if (fatent->entry & 1)
 		next = (*ent12_p[0] >> 4) | (*ent12_p[1] << 4);
 	else
 		next = (*ent12_p[1] << 8) | *ent12_p[0];
+	spin_unlock(&fat12_entry_lock);
+
 	next &= 0x0fff;
 	if (next >= BAD_FAT12)
 		next = FAT_ENT_EOF;
@@ -151,6 +156,7 @@ static void fat12_ent_put(struct fat_entry *fatent, int new)
 	if (new == FAT_ENT_EOF)
 		new = EOF_FAT12;
 
+	spin_lock(&fat12_entry_lock);
 	if (fatent->entry & 1) {
 		*ent12_p[0] = (new << 4) | (*ent12_p[0] & 0x0f);
 		*ent12_p[1] = new >> 4;
@@ -158,6 +164,7 @@ static void fat12_ent_put(struct fat_entry *fatent, int new)
 		*ent12_p[0] = new & 0xff;
 		*ent12_p[1] = (*ent12_p[1] & 0xf0) | (new >> 8);
 	}
+	spin_unlock(&fat12_entry_lock);
 
 	mark_buffer_dirty(fatent->bhs[0]);
 	if (fatent->nr_bhs == 2)