Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit a5c46453 authored by Christian Brauner's avatar Christian Brauner Committed by Greg Kroah-Hartman
Browse files

fs: better handle deep ancestor chains in is_subdir() 



[ Upstream commit 391b59b045004d5b985d033263ccba3e941a7740 ]

Jan reported that 'cd ..' may take a long time in deep directory
hierarchies under a bind-mount. If concurrent renames happen it is
possible to livelock in is_subdir() because it will keep retrying.

Change is_subdir() from simply retrying over and over to retry once and
then acquire the rename lock to handle deep ancestor chains better. The
list of alternatives to this approach were less then pleasant. Change
the scope of rcu lock to cover the whole walk while at it.

A big thanks to Jan and Linus. Both Jan and Linus had proposed
effectively the same thing just that one version ended up being slightly
more elegant.

Reported-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 48542881

fs/dcache.c

+14 −17
Original line number Original line Diff line number Diff line
@@ -3003,28 +3003,25 @@ EXPORT_SYMBOL(d_splice_alias); 
  

  

bool is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)

bool is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)

{

{

	bool result;

	bool subdir;

	unsigned seq;

	unsigned seq;





	if (new_dentry == old_dentry)

	if (new_dentry == old_dentry)

		return true;

		return true;





	do {

	/* Access d_parent under rcu as d_move() may change it. */

		/* for restarting inner loop in case of seq retry */

		seq = read_seqbegin(&rename_lock);

		/*

		 * Need rcu_readlock to protect against the d_parent trashing

		 * due to d_move

		 */

	rcu_read_lock();

	rcu_read_lock();

		if (d_ancestor(old_dentry, new_dentry))

	seq = read_seqbegin(&rename_lock);

			result = true;

	subdir = d_ancestor(old_dentry, new_dentry);

		else

	 /* Try lockless once... */

			result = false;

	if (read_seqretry(&rename_lock, seq)) {

		/* ...else acquire lock for progress even on deep chains. */

		read_seqlock_excl(&rename_lock);

		subdir = d_ancestor(old_dentry, new_dentry);

		read_sequnlock_excl(&rename_lock);

	}

	rcu_read_unlock();

	rcu_read_unlock();

	} while (read_seqretry(&rename_lock, seq));

	return subdir;



	return result;

}

}

EXPORT_SYMBOL(is_subdir);

EXPORT_SYMBOL(is_subdir);