From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?UTF-8?q?Michal=20Koutn=C3=BD?= Subject: [PATCH] cgroup: Reorganize css_set_lock and kernfs path processing Date: Mon, 5 Sep 2022 19:09:44 +0200 Message-ID: <20220905170944.23071-1-mkoutny@suse.com> Mime-Version: 1.0 Content-Transfer-Encoding: base64 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1662397803; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=3OBVlzrdocXXXTKTLgX42mUjjVVDDyYNMpF+eKmmtCQ=; b=TI8brioUhcbTj37ERGLmMeQ2UJM6oYa0WDG5ztQJfuPzEBQ12TugPu5sLcrLplpMvQ7eS/ Kud9kF71PFFvZndsFipFQv52uLTLbx3TMiCwxg4XR2m3pPmsrHu2tcrH66+Wrp5CFYLIC0 uejAfimdJr9AN0+Uw9yU/5qKSRtuBfk= List-ID: Content-Type: text/plain; charset="macroman" To: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Cc: Tejun Heo , Zefan Li , Johannes Weiner 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linux-kernel@vger.kernel.org Cc: Tejun Heo , Zefan Li , Johannes Weiner , Dan Carpenter Subject: [PATCH] cgroup: Reorganize css_set_lock and kernfs path processing Date: Mon, 5 Sep 2022 19:09:44 +0200 Message-Id: <20220905170944.23071-1-mkoutny@suse.com> X-Mailer: git-send-email 2.37.0 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The commit 74e4b956eb1c incorrectly wrapped kernfs_walk_and_get (might_sleep) under css_set_lock (spinlock). css_set_lock is needed by __cset_cgroup_from_root to ensure stable cset->cgrp_links. The returned cgroup object is pinned by the css_set (*). Because current cannot switch namespace asynchronously, the css_set is also pinned by ns_proxy->cgroup_ns (regardless of current's cgroup migration). Kernfs code that traverses paths with relative root_cgroup not need css_set_lock. (*) Except for root cgroups. The default hierarchy root (under which cgroup id and path resolution only happens) is eternal so it's moot. cgroup_show_path (VFS callback) is expected to be synchronized (**) wrt kill_sb (VFS callback) (mnt_namespace.list with namespace_sem). (**) If not, it's still an independent issue from this and the fixed one. Fixes: 74e4b956eb1c: ("cgroup: Honor caller's cgroup NS when resolving path") Reported-by: Dan Carpenter Signed-off-by: Michal Koutný --- kernel/cgroup/cgroup.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) I considered adding get_cgroup() into current_cgns_cgroup_from_root to avoid reliance on the transitive pinning via css_set. After reasoning about no asynchronous NS switch and v1 hiearchies kill_sb it didn't seem to bring that much benefit (it didn't compose well with BUG_ON(!cgrp) neither). diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index e0b72eb5d283..8c9497f01332 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -1391,11 +1391,16 @@ static void cgroup_destroy_root(struct cgroup_root *root) cgroup_free_root(root); } +/* + * Returned cgroup is without refcount but it's valid as long as cset pins it. + */ static inline struct cgroup *__cset_cgroup_from_root(struct css_set *cset, struct cgroup_root *root) { struct cgroup *res_cgroup = NULL; + lockdep_assert_held(&css_set_lock); + if (cset == &init_css_set) { res_cgroup = &root->cgrp; } else if (root == &cgrp_dfl_root) { @@ -1426,8 +1431,6 @@ current_cgns_cgroup_from_root(struct cgroup_root *root) struct cgroup *res = NULL; struct css_set *cset; - lockdep_assert_held(&css_set_lock); - rcu_read_lock(); cset = current->nsproxy->cgroup_ns->root_cset; @@ -1446,7 +1449,6 @@ static struct cgroup *cset_cgroup_from_root(struct css_set *cset, struct cgroup *res = NULL; lockdep_assert_held(&cgroup_mutex); - lockdep_assert_held(&css_set_lock); res = __cset_cgroup_from_root(cset, root); @@ -1861,8 +1863,8 @@ int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node, spin_lock_irq(&css_set_lock); ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot); - len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX); spin_unlock_irq(&css_set_lock); + len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX); if (len >= PATH_MAX) len = -ERANGE; @@ -6649,8 +6651,8 @@ struct cgroup *cgroup_get_from_path(const char *path) spin_lock_irq(&css_set_lock); root_cgrp = current_cgns_cgroup_from_root(&cgrp_dfl_root); - kn = kernfs_walk_and_get(root_cgrp->kn, path); spin_unlock_irq(&css_set_lock); + kn = kernfs_walk_and_get(root_cgrp->kn, path); if (!kn) goto out; base-commit: a8c52eba880a6e8c07fc2130604f8e386b90b763 -- 2.37.0