NFS: Fix nfs_file_llseek()

After the BKL removal patches were applied to the rest of the NFS code, the
BKL protection in nfs_file_llseek() is no longer sufficient to ensure that
inode->i_size is read safely in generic_file_llseek_unlocked().

In order to fix the situation, we either have to replace the naked read of
inode->i_size in generic_file_llseek_unlocked() with i_size_read(), or the
whole thing needs to be executed under the inode->i_lock;
In order to avoid disrupting other filesystems, avoid touching
generic_file_llseek_unlocked() for now...
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/file.c

@@ -188,13 +188,16 @@ static loff_t nfs_file_llseek(struct file *filp, loff_t offset, int origin)
 	/* origin == SEEK_END => we must revalidate the cached file length */
 	if (origin == SEEK_END) {
 		struct inode *inode = filp->f_mapping->host;
+
 		int retval = nfs_revalidate_file_size(inode, filp);
 		if (retval < 0)
 			return (loff_t)retval;
-	}
-	lock_kernel();	/* BKL needed? */
-	loff = generic_file_llseek_unlocked(filp, offset, origin);
-	unlock_kernel();
+
+		spin_lock(&inode->i_lock);
+		loff = generic_file_llseek_unlocked(filp, offset, origin);
+		spin_unlock(&inode->i_lock);
+	} else
+		loff = generic_file_llseek_unlocked(filp, offset, origin);
 	return loff;
 }