Re: [PATCH v3 03/10] fs: lockless mntns rbtree lookup

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2024-12-13 at 00:03 +0100, Christian Brauner wrote:
> Currently we use a read-write lock but for the simple search case we can
> make this lockless. Creating a new mount namespace is a rather rare
> event compared with querying mounts in a foreign mount namespace. Once
> this is picked up by e.g., systemd to list mounts in another mount in
> it's isolated services or in containers this will be used a lot so this
> seems worthwhile doing.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/mount.h     |   5 ++-
>  fs/namespace.c | 119 +++++++++++++++++++++++++++++++++++----------------------
>  2 files changed, 77 insertions(+), 47 deletions(-)
> 
> diff --git a/fs/mount.h b/fs/mount.h
> index 185fc56afc13338f8185fe818051444d540cbd5b..36ead0e45e8aa7614c00001102563a711d9dae6e 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -12,7 +12,10 @@ struct mnt_namespace {
>  	struct user_namespace	*user_ns;
>  	struct ucounts		*ucounts;
>  	u64			seq;	/* Sequence number to prevent loops */
> -	wait_queue_head_t poll;
> +	union {
> +		wait_queue_head_t	poll;
> +		struct rcu_head		mnt_ns_rcu;
> +	};
>  	u64 event;
>  	unsigned int		nr_mounts; /* # of mounts in the namespace */
>  	unsigned int		pending_mounts;
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 10fa18dd66018fadfdc9d18c59a851eed7bd55ad..52adee787eb1b6ee8831705b2b121854c3370fb3 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -79,6 +79,8 @@ static DECLARE_RWSEM(namespace_sem);
>  static HLIST_HEAD(unmounted);	/* protected by namespace_sem */
>  static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
>  static DEFINE_RWLOCK(mnt_ns_tree_lock);
> +static seqcount_rwlock_t mnt_ns_tree_seqcount = SEQCNT_RWLOCK_ZERO(mnt_ns_tree_seqcount, &mnt_ns_tree_lock);
> +
>  static struct rb_root mnt_ns_tree = RB_ROOT; /* protected by mnt_ns_tree_lock */
>  
>  struct mount_kattr {
> @@ -105,17 +107,6 @@ EXPORT_SYMBOL_GPL(fs_kobj);
>   */
>  __cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);
>  
> -static int mnt_ns_cmp(u64 seq, const struct mnt_namespace *ns)
> -{
> -	u64 seq_b = ns->seq;
> -
> -	if (seq < seq_b)
> -		return -1;
> -	if (seq > seq_b)
> -		return 1;
> -	return 0;
> -}
> -
>  static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
>  {
>  	if (!node)
> @@ -123,19 +114,41 @@ static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
>  	return rb_entry(node, struct mnt_namespace, mnt_ns_tree_node);
>  }
>  
> -static bool mnt_ns_less(struct rb_node *a, const struct rb_node *b)
> +static int mnt_ns_cmp(struct rb_node *a, const struct rb_node *b)
>  {
>  	struct mnt_namespace *ns_a = node_to_mnt_ns(a);
>  	struct mnt_namespace *ns_b = node_to_mnt_ns(b);
>  	u64 seq_a = ns_a->seq;
> +	u64 seq_b = ns_b->seq;
> +
> +	if (seq_a < seq_b)
> +		return -1;
> +	if (seq_a > seq_b)
> +		return 1;
> +	return 0;
> +}
>  
> -	return mnt_ns_cmp(seq_a, ns_b) < 0;
> +static inline void mnt_ns_tree_write_lock(void)
> +{
> +	write_lock(&mnt_ns_tree_lock);
> +	write_seqcount_begin(&mnt_ns_tree_seqcount);
> +}
> +
> +static inline void mnt_ns_tree_write_unlock(void)
> +{
> +	write_seqcount_end(&mnt_ns_tree_seqcount);
> +	write_unlock(&mnt_ns_tree_lock);
>  }
>  
>  static void mnt_ns_tree_add(struct mnt_namespace *ns)
>  {
> -	guard(write_lock)(&mnt_ns_tree_lock);
> -	rb_add(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_less);
> +	struct rb_node *node;
> +
> +	mnt_ns_tree_write_lock();
> +	node = rb_find_add_rcu(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_cmp);
> +	mnt_ns_tree_write_unlock();
> +
> +	WARN_ON_ONCE(node);
>  }
>  
>  static void mnt_ns_release(struct mnt_namespace *ns)
> @@ -150,41 +163,36 @@ static void mnt_ns_release(struct mnt_namespace *ns)
>  }
>  DEFINE_FREE(mnt_ns_release, struct mnt_namespace *, if (_T) mnt_ns_release(_T))
>  
> +static void mnt_ns_release_rcu(struct rcu_head *rcu)
> +{
> +	struct mnt_namespace *mnt_ns;
> +
> +	mnt_ns = container_of(rcu, struct mnt_namespace, mnt_ns_rcu);
> +	mnt_ns_release(mnt_ns);
> +}
> +
>  static void mnt_ns_tree_remove(struct mnt_namespace *ns)
>  {
>  	/* remove from global mount namespace list */
>  	if (!is_anon_ns(ns)) {
> -		guard(write_lock)(&mnt_ns_tree_lock);
> +		mnt_ns_tree_write_lock();
>  		rb_erase(&ns->mnt_ns_tree_node, &mnt_ns_tree);
> +		mnt_ns_tree_write_unlock();
>  	}
>  
> -	mnt_ns_release(ns);
> +	call_rcu(&ns->mnt_ns_rcu, mnt_ns_release_rcu);
>  }
>  
> -/*
> - * Returns the mount namespace which either has the specified id, or has the
> - * next smallest id afer the specified one.
> - */
> -static struct mnt_namespace *mnt_ns_find_id_at(u64 mnt_ns_id)
> +static int mnt_ns_find(const void *key, const struct rb_node *node)
>  {
> -	struct rb_node *node = mnt_ns_tree.rb_node;
> -	struct mnt_namespace *ret = NULL;
> -
> -	lockdep_assert_held(&mnt_ns_tree_lock);
> -
> -	while (node) {
> -		struct mnt_namespace *n = node_to_mnt_ns(node);
> +	const u64 mnt_ns_id = *(u64 *)key;
> +	const struct mnt_namespace *ns = node_to_mnt_ns(node);
>  
> -		if (mnt_ns_id <= n->seq) {
> -			ret = node_to_mnt_ns(node);
> -			if (mnt_ns_id == n->seq)
> -				break;
> -			node = node->rb_left;
> -		} else {
> -			node = node->rb_right;
> -		}
> -	}
> -	return ret;
> +	if (mnt_ns_id < ns->seq)
> +		return -1;
> +	if (mnt_ns_id > ns->seq)
> +		return 1;
> +	return 0;
>  }
>  
>  /*
> @@ -194,18 +202,37 @@ static struct mnt_namespace *mnt_ns_find_id_at(u64 mnt_ns_id)
>   * namespace the @namespace_sem must first be acquired. If the namespace has
>   * already shut down before acquiring @namespace_sem, {list,stat}mount() will
>   * see that the mount rbtree of the namespace is empty.
> + *
> + * Note the lookup is lockless protected by a sequence counter. We only
> + * need to guard against false negatives as false positives aren't
> + * possible. So if we didn't find a mount namespace and the sequence
> + * counter has changed we need to retry. If the sequence counter is
> + * still the same we know the search actually failed.
>   */
>  static struct mnt_namespace *lookup_mnt_ns(u64 mnt_ns_id)
>  {
> -       struct mnt_namespace *ns;
> +	struct mnt_namespace *ns;
> +	struct rb_node *node;
> +	unsigned int seq;
> +
> +	guard(rcu)();
> +	do {
> +		seq = read_seqcount_begin(&mnt_ns_tree_seqcount);
> +		node = rb_find_rcu(&mnt_ns_id, &mnt_ns_tree, mnt_ns_find);
> +		if (node)
> +			break;
> +	} while (read_seqcount_retry(&mnt_ns_tree_seqcount, seq));
>  
> -       guard(read_lock)(&mnt_ns_tree_lock);
> -       ns = mnt_ns_find_id_at(mnt_ns_id);
> -       if (!ns || ns->seq != mnt_ns_id)
> -               return NULL;
> +	if (!node)
> +		return NULL;
>  
> -       refcount_inc(&ns->passive);
> -       return ns;
> +	/*
> +	 * The last reference count is put with RCU delay so we can
> +	 * unconditonally acquire a reference here.
> +	 */
> +	ns = node_to_mnt_ns(node);
> +	refcount_inc(&ns->passive);

I'm a little uneasy with the unconditional refcount_inc() here. It
seems quite possible that this could to a 0->1 transition here. You may
be right that that technically won't cause a problem with the rcu lock
held, but at the very least, that will cause a refcount warning to pop.

Maybe this should be a refcount_inc_not_zero() and then you just return
NULL if the increment doesn't occur?

> +	return ns;
>  }
>  
>  static inline void lock_mount_hash(void)
> 

-- 
Jeff Layton <jlayton@kernel.org>