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>

1 files changed, 19 insertions, 2 deletions

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 9e37ad93c730..bdd8fb7be2ca 100644
--- a/fs/fat/inode.c
+++ b/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);