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SubjectRe: [RFC PATCH 2/3] cgroup: Rely on namespace_sem in current_cgns_cgroup_from_root explicitly
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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 <mkoutny@suse.com>
> ---
> 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

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