From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [RFC PATCH 2/3] cgroup: Rely on namespace_sem in current_cgns_cgroup_from_root explicitly Date: Tue, 2 May 2023 15:50:59 -0400 Message-ID: <0beea9a5-a163-75e2-59c2-2b092fe96d16@redhat.com> References: <20230502133847.14570-1-mkoutny@suse.com> <20230502133847.14570-3-mkoutny@suse.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1683057064; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0u1c/zI7EG5oA8EEQf+n5SVUWPETfw09W7j85YlQyd0=; b=IdzIcJ/kRFV1yYUK4HIdiN69G4b2bLARjoPAnaQ1xn9u9Q2DWxMZp8ReVuiicJ28rJ/qfU 7+nLBevIfYZujYs18MSsrMXD6yukaQ1642CzeiN4ghkYgwwDfDxkCe9JpQyTuAHQint6Z6 Ihm8+0KYCvdeqcW1wll8kzBlF/dYy8A= Content-Language: en-US In-Reply-To: <20230502133847.14570-3-mkoutny@suse.com> List-ID: Content-Type: text/plain; charset="utf-8"; format="flowed" To: =?UTF-8?Q?Michal_Koutn=c3=bd?= , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Cc: Alexander Viro , Christian Brauner , Tejun Heo , Zefan Li , Johannes Weiner , Dave Chinner , Rik van Riel , Jiri Wiesner On 5/2/23 09:38, Michal Koutný wrote: > The function current_cgns_cgroup_from_root() expects a stable > cgroup_root, which is currently ensured with RCU read side paired with > cgroup_destroy_root() called after RCU period. > > The particular current_cgns_cgroup_from_root() is called from VFS code > and cgroup_root stability can be also ensured by namespace_sem. Mark it > explicitly as a preparation for further rework. > > Signed-off-by: Michal Koutný > --- > fs/namespace.c | 5 ++++- > include/linux/mount.h | 4 ++++ > kernel/cgroup/cgroup.c | 7 +++---- > 3 files changed, 11 insertions(+), 5 deletions(-) > > diff --git a/fs/namespace.c b/fs/namespace.c > index 54847db5b819..0d2333832064 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -71,7 +71,10 @@ static DEFINE_IDA(mnt_group_ida); > static struct hlist_head *mount_hashtable __read_mostly; > static struct hlist_head *mountpoint_hashtable __read_mostly; > static struct kmem_cache *mnt_cache __read_mostly; > -static DECLARE_RWSEM(namespace_sem); > +DECLARE_RWSEM(namespace_sem); > +#ifdef CONFIG_LOCKDEP > +EXPORT_SYMBOL_GPL(namespace_sem); I don't think fs/namespace.o and kernel/cgroup/cgroup.o can't be built into a kernel module. I doubt we need to export it. > +#endif > static HLIST_HEAD(unmounted); /* protected by namespace_sem */ > static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */ > > diff --git a/include/linux/mount.h b/include/linux/mount.h > index 1ea326c368f7..6277435f6748 100644 > --- a/include/linux/mount.h > +++ b/include/linux/mount.h > @@ -80,6 +80,10 @@ static inline struct mnt_idmap *mnt_idmap(const struct vfsmount *mnt) > return smp_load_acquire(&mnt->mnt_idmap); > } > > +#ifdef CONFIG_LOCKDEP > +extern struct rw_semaphore namespace_sem; > +#endif > + > extern int mnt_want_write(struct vfsmount *mnt); > extern int mnt_want_write_file(struct file *file); > extern void mnt_drop_write(struct vfsmount *mnt); > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c > index 55e5f0110e3b..32d693a797b9 100644 > --- a/kernel/cgroup/cgroup.c > +++ b/kernel/cgroup/cgroup.c > @@ -1440,13 +1440,12 @@ current_cgns_cgroup_from_root(struct cgroup_root *root) > > lockdep_assert_held(&css_set_lock); > > - rcu_read_lock(); > + /* namespace_sem ensures `root` stability on unmount */ > + lockdep_assert(lockdep_is_held_type(&namespace_sem, -1)); It will be easier if you just use lockdep_is_held() without the 2nd argment. > > cset = current->nsproxy->cgroup_ns->root_cset; > res = __cset_cgroup_from_root(cset, root); > > - rcu_read_unlock(); > - > return res; > } > > @@ -1454,7 +1453,7 @@ current_cgns_cgroup_from_root(struct cgroup_root *root) > * Look up cgroup associated with current task's cgroup namespace on the default > * hierarchy. > * > - * Unlike current_cgns_cgroup_from_root(), this doesn't need locks: > + * Relaxed locking requirements: > * - Internal rcu_read_lock is unnecessary because we don't dereference any rcu > * pointers. > * - css_set_lock is not needed because we just read cset->dfl_cgrp. Cheers, Longman