vfs: prevent remount read-only if pending removes

If there are any inodes on the super block that have been unlinked
(i_nlink == 0) but have not yet been deleted then prevent the
remounting the super block read-only.
Reported-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/file_table.c

@@ -474,29 +474,6 @@ void file_sb_list_del(struct file *file)
 
 #endif
 
-int fs_may_remount_ro(struct super_block *sb)
-{
-	struct file *file;
-	/* Check that no files are currently opened for writing. */
-	lg_global_lock(files_lglock);
-	do_file_list_for_each_entry(sb, file) {
-		struct inode *inode = file->f_path.dentry->d_inode;
-
-		/* File with pending delete? */
-		if (inode->i_nlink == 0)
-			goto too_bad;
-
-		/* Writeable file? */
-		if (S_ISREG(inode->i_mode) && (file->f_mode & FMODE_WRITE))
-			goto too_bad;
-	} while_file_list_for_each_entry;
-	lg_global_unlock(files_lglock);
-	return 1; /* Tis' cool bro. */
-too_bad:
-	lg_global_unlock(files_lglock);
-	return 0;
-}
-
 /**
  *	mark_files_ro - mark all files read-only
  *	@sb: superblock in question

fs/namespace.c

@@ -449,6 +449,10 @@ int sb_prepare_remount_readonly(struct super_block *sb)
 	struct mount *mnt;
 	int err = 0;
 
+	/* Racy optimization.  Recheck the counter under MNT_WRITE_HOLD */
+	if (atomic_long_read(&sb->s_remove_count))
+		return -EBUSY;
+
 	br_write_lock(vfsmount_lock);
 	list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
 		if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
@@ -460,6 +464,9 @@ int sb_prepare_remount_readonly(struct super_block *sb)
 			}
 		}
 	}
+	if (!err && atomic_long_read(&sb->s_remove_count))
+		err = -EBUSY;
+
 	if (!err) {
 		sb->s_readonly_remount = 1;
 		smp_wmb();

fs/super.c

@@ -729,10 +729,6 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 			retval = sb_prepare_remount_readonly(sb);
 			if (retval)
 				return retval;
-
-			retval = -EBUSY;
-			if (!fs_may_remount_ro(sb))
-				goto cancel_readonly;
 		}
 	}
 

include/linux/fs.h

@@ -2150,8 +2150,6 @@ extern const struct file_operations read_pipefifo_fops;
 extern const struct file_operations write_pipefifo_fops;
 extern const struct file_operations rdwr_pipefifo_fops;
 
-extern int fs_may_remount_ro(struct super_block *);
-
 #ifdef CONFIG_BLOCK
 /*
  * return READ, READA, or WRITE