fat: ->i_pos race fix

i_pos is 64bits value, hence it's not atomic to update.

Important place is fat_write_inode() only, other places without lock
are just for printk().

This adds lock for "BITS_PER_LONG == 32" kernel.
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fat/inode.c

@@ -542,6 +542,20 @@ static int fat_statfs(struct dentry *dentry, struct kstatfs *buf)
 	return 0;
 }
 
+static inline loff_t fat_i_pos_read(struct msdos_sb_info *sbi,
+				    struct inode *inode)
+{
+	loff_t i_pos;
+#if BITS_PER_LONG == 32
+	spin_lock(&sbi->inode_hash_lock);
+#endif
+	i_pos = MSDOS_I(inode)->i_pos;
+#if BITS_PER_LONG == 32
+	spin_unlock(&sbi->inode_hash_lock);
+#endif
+	return i_pos;
+}
+
 static int fat_write_inode(struct inode *inode, int wait)
 {
 	struct super_block *sb = inode->i_sb;
@@ -551,9 +565,12 @@ static int fat_write_inode(struct inode *inode, int wait)
 	loff_t i_pos;
 	int err;
 
+	if (inode->i_ino == MSDOS_ROOT_INO)
+		return 0;
+
 retry:
-	i_pos = MSDOS_I(inode)->i_pos;
-	if (inode->i_ino == MSDOS_ROOT_INO || !i_pos)
+	i_pos = fat_i_pos_read(sbi, inode);
+	if (!i_pos)
 		return 0;
 
 	bh = sb_bread(sb, i_pos >> sbi->dir_per_block_bits);