From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8CB3D1E0E10 for ; Fri, 13 Dec 2024 14:11:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734099103; cv=none; b=h4Umw2THp548YxvhG5Eyu6SlZ0nLnAeEFAohiDtdT+RXeVKThD1lnTkc3gHn8hUTiDF9Xi7ZmLr1IDaNRmsExOYKgxNylvRD87Rz+p1vue+BZOtMhvPxY5vfZI8J4Gr5rLTiYJ7wyM4ircum5yrOsC/Z+AGIl5u+xWUgs48RafU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734099103; c=relaxed/simple; bh=Qq23nloZ4h7NrunmteXDHG+tm8kZ9iCfCEIK2EDzTBI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=j3D+woVvrKZ5VyLzDpKK4OVeRUgmYdcBNqswE4T8T4dfxnlq3OfJfDkwM4gj+Zm+afd7txZ0O8714h41Ia6UZh6hEbwI4J5B3TVf1QGEXWvVSU3mxazwDcFM9wsTi6Na6Kfw4cvgVY1ffpOwE2GJ2Yck6B9L5Bs7hFtadaCh4nM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=c6vnN5sG; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="c6vnN5sG" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9607EC4CED0; Fri, 13 Dec 2024 14:11:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1734099103; bh=Qq23nloZ4h7NrunmteXDHG+tm8kZ9iCfCEIK2EDzTBI=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=c6vnN5sGnJzkI+qaKDLOM8bCujtP8c0Cgf2iARaLcyCcxknj+AdiAw9UCk/faICbQ 5UuqK2GzzYDxEsRipKVhn9Gkbw4wkWydZXqqYFfEI7ZfrRlbCs98KWrZ+xNnsLy4LY OR4tvJHohzYAXbSVOhS2yiGk1eaj4GXffB6nD5zS94b/6+/e+Vjy5/ZG/Zba0FufYq ku2nAsEx6kid3gFq9wi1QUsmzGz+w03gom7h5ritTrP9HADSyaXMGVDHznc24jZUfY Lo7JgnWw6MqJolR1v7OxbKioVp3gpFFoxFDRm2QMLc75fY9e/Gv0otf5PQJtH7pFj2 0ZXMNCjSTYgFg== Message-ID: Subject: Re: [PATCH v3 03/10] fs: lockless mntns rbtree lookup From: Jeff Layton To: Christian Brauner , Josef Bacik Cc: "Paul E. McKenney" , Peter Ziljstra , linux-fsdevel@vger.kernel.org Date: Fri, 13 Dec 2024 09:11:41 -0500 In-Reply-To: <20241213-work-mount-rbtree-lockless-v3-3-6e3cdaf9b280@kernel.org> References: <20241213-work-mount-rbtree-lockless-v3-0-6e3cdaf9b280@kernel.org> <20241213-work-mount-rbtree-lockless-v3-3-6e3cdaf9b280@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ 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CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.52.4 (3.52.4-2.fc40) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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. >=20 > Signed-off-by: Christian Brauner > --- > fs/mount.h | 5 ++- > fs/namespace.c | 119 +++++++++++++++++++++++++++++++++++----------------= ------ > 2 files changed, 77 insertions(+), 47 deletions(-) >=20 > diff --git a/fs/mount.h b/fs/mount.h > index 185fc56afc13338f8185fe818051444d540cbd5b..36ead0e45e8aa7614c0000110= 2563a711d9dae6e 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..52adee787eb1b6ee8831705b2= b121854c3370fb3 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 =3D SEQCNT_RWLOCK_ZERO(mnt= _ns_tree_seqcount, &mnt_ns_tree_lock); > + > static struct rb_root mnt_ns_tree =3D RB_ROOT; /* protected by mnt_ns_tr= ee_lock */ > =20 > struct mount_kattr { > @@ -105,17 +107,6 @@ EXPORT_SYMBOL_GPL(fs_kobj); > */ > __cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock); > =20 > -static int mnt_ns_cmp(u64 seq, const struct mnt_namespace *ns) > -{ > - u64 seq_b =3D 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); > } > =20 > -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 =3D node_to_mnt_ns(a); > struct mnt_namespace *ns_b =3D node_to_mnt_ns(b); > u64 seq_a =3D ns_a->seq; > + u64 seq_b =3D ns_b->seq; > + > + if (seq_a < seq_b) > + return -1; > + if (seq_a > seq_b) > + return 1; > + return 0; > +} > =20 > - 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); > } > =20 > 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 =3D rb_find_add_rcu(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_cm= p); > + mnt_ns_tree_write_unlock(); > + > + WARN_ON_ONCE(node); > } > =20 > 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_relea= se(_T)) > =20 > +static void mnt_ns_release_rcu(struct rcu_head *rcu) > +{ > + struct mnt_namespace *mnt_ns; > + > + mnt_ns =3D 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(); > } > =20 > - mnt_ns_release(ns); > + call_rcu(&ns->mnt_ns_rcu, mnt_ns_release_rcu); > } > =20 > -/* > - * 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 =3D mnt_ns_tree.rb_node; > - struct mnt_namespace *ret =3D NULL; > - > - lockdep_assert_held(&mnt_ns_tree_lock); > - > - while (node) { > - struct mnt_namespace *n =3D node_to_mnt_ns(node); > + const u64 mnt_ns_id =3D *(u64 *)key; > + const struct mnt_namespace *ns =3D node_to_mnt_ns(node); > =20 > - if (mnt_ns_id <=3D n->seq) { > - ret =3D node_to_mnt_ns(node); > - if (mnt_ns_id =3D=3D n->seq) > - break; > - node =3D node->rb_left; > - } else { > - node =3D node->rb_right; > - } > - } > - return ret; > + if (mnt_ns_id < ns->seq) > + return -1; > + if (mnt_ns_id > ns->seq) > + return 1; > + return 0; > } > =20 > /* > @@ -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 =3D read_seqcount_begin(&mnt_ns_tree_seqcount); > + node =3D rb_find_rcu(&mnt_ns_id, &mnt_ns_tree, mnt_ns_find); > + if (node) > + break; > + } while (read_seqcount_retry(&mnt_ns_tree_seqcount, seq)); > =20 > - guard(read_lock)(&mnt_ns_tree_lock); > - ns =3D mnt_ns_find_id_at(mnt_ns_id); > - if (!ns || ns->seq !=3D mnt_ns_id) > - return NULL; > + if (!node) > + return NULL; > =20 > - refcount_inc(&ns->passive); > - return ns; > + /* > + * The last reference count is put with RCU delay so we can > + * unconditonally acquire a reference here. > + */ > + ns =3D 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; > } > =20 > static inline void lock_mount_hash(void) >=20 --=20 Jeff Layton