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 BD1A0212D74 for ; Thu, 12 Dec 2024 12:15:23 +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=1734005723; cv=none; b=jsSdjHbPEYEhyDLOIu3UUGTPpoEMZbnlQUHOf5GM1vwcYqdDAfK5CvBcqmfHlLoTKyH6j7WP2eDCVi+b23QtNfGvlmHY0jF41B92uBsPixIhoIiTRe4xBv8Yc7VKKXvLMdZTioQE9lQHRwUypRGnCxUEts/molIfEBAqcF2xWWw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734005723; c=relaxed/simple; bh=SpLeiNuXm82rxIWAZncCDxJqXIP+2NCT+SLB7P3HhME=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=N3fjc6177xtalrsmX4QuhgQubtdBi4tPU0UmyaqCarn6t6YojomclfI6dtAaQcAEC3TJ6vzFjmPuf/sHnNna+lspKBuUqHESotJL8i83pkLH22y+NPgnuQTj9RNMz2yiW4E8qTkbcNoKTnJIEpZkMmLZV/WKpxeEEgHXeUOimhE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JljVJuSG; 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="JljVJuSG" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C7D68C4CED4; Thu, 12 Dec 2024 12:15:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1734005723; bh=SpLeiNuXm82rxIWAZncCDxJqXIP+2NCT+SLB7P3HhME=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=JljVJuSGqH16kg3clCuyjQ2PDPES4lQXTY8dAJ1phbE4J9bg17Q2i3tYjSKHF6QIM JqXMfo6K6tuzWdEiLUBeIguMoE7JKrxVWQ0PHH0D8COSqaa+NQBlXybkHrsDAcwYvm 9BBUx7CbFN7S3LMGv/HqNEJcJqhFRvfKVPWz0kuSTlmTStIv9sJAzITRuwcc/D78M6 D4Xf9IhkR7q08ZNiMGS/kOzEa7vqcDbl5JwijgsxkVjqFJGYGRhH5vuHdZCIGBf1P5 uxrosTJXMxjZ5mJrNArjRI6/tu046yk1x2Vn7vGZ4Zup7Y9PD7cuf3Eyoa51/hKCV1 k5UqnsMh448JQ== Message-ID: <01b7d0df19957da6d628dbe4390e086c51bf5a12.camel@kernel.org> Subject: Re: [PATCH v2 3/8] 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: Thu, 12 Dec 2024 07:15:21 -0500 In-Reply-To: <20241212-work-mount-rbtree-lockless-v2-3-4fe6cef02534@kernel.org> References: <20241212-work-mount-rbtree-lockless-v2-0-4fe6cef02534@kernel.org> <20241212-work-mount-rbtree-lockless-v2-3-4fe6cef02534@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 Thu, 2024-12-12 at 12:56 +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..3c3763d8ae821d6a117c52880= 8dbc94d0251f964 100644 > --- a/fs/mount.h > +++ b/fs/mount.h > @@ -16,7 +16,10 @@ struct mnt_namespace { > u64 event; > unsigned int nr_mounts; /* # of mounts in the namespace */ > unsigned int pending_mounts; > - struct rb_node mnt_ns_tree_node; /* node in the mnt_ns_tree */ > + union { > + struct rb_node mnt_ns_tree_node; /* node in the mnt_ns_tree */ > + struct rcu_head mnt_ns_rcu; > + }; I second Peter's concerns about the union here. You really want to union the rcu structure with something that isn't ever touched during a search for the object. > refcount_t passive; /* number references not pinning @mounts */ > } __randomize_layout; > =20 > diff --git a/fs/namespace.c b/fs/namespace.c > index 10fa18dd66018fadfdc9d18c59a851eed7bd55ad..9463b9ab95f0a5db32cfe5fc5= 564d7f25ce3e06f 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)); I don't really get the need for a seqcount here. Typically we use those when you could potentially find an object that's in some sort of inconsistent state. Is that the case here? It seems like you'd either find the object or not. It seems like either outcome is OK without the need to retry the search? > =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 after RCU delay so we > + * don't need to use refcount_inc_not_zero(). > + */ > + ns =3D node_to_mnt_ns(node); > + refcount_inc(&ns->passive); > + return ns; > } > =20 > static inline void lock_mount_hash(void) >=20 --=20 Jeff Layton