On Fri, 2024-12-13 at 00:03 +0100, Christian Brauner wrote:
> Hey,
>
> This now also includes selftests for iterating mount namespaces both
> backwards and forwards.
>
> Currently we take the read lock when looking for a mount namespace to
> list mounts in. We can make this lockless. The simple search case can
> just use a sequence counter to detect concurrent changes to the rbtree.
>
> For walking the list of mount namespaces sequentially via nsfs we keep a
> separate rcu list as rb_prev() and rb_next() aren't usable safely with
> rcu.
>
> Since creating mount namespaces is a relatively rare event compared with
> querying mounts in a foreign mount namespace this is worth it. Once
> libmount and systemd pick up this mechanism to list mounts in foreign
> mount namespaces this will be used very frequently.
>
> Thanks!
> Christian
>
> ---
> Changes in v3:
> - Add selftests.
> - Put list_head into a union with the wait_queue_head_t for poll instead
> of the mnt_ns_tree_node which would've risked breaking rbtree
> traversal.
> - Handle insertion into the mount namespace list correctly by making use
> of the rbtree position information after the mount namespace has been
> added to it.
> - Improve the documentation for the new list_bidir_{del,prev}_rcu().
> - Link to v2: https://lore.kernel.org/r/20241212-work-mount-rbtree-lockless-v2-0-4fe6cef02534@xxxxxxxxxx
>
> Changes in v2:
> - Remove mnt_ns_find_it_at() by switching to rb_find_rcu().
> - Add separate list to lookup sequential mount namespaces.
> - Link to v1: https://lore.kernel.org/r/20241210-work-mount-rbtree-lockless-v1-0-338366b9bbe4@xxxxxxxxxx
>
> ---
> Christian Brauner (10):
> mount: remove inlude/nospec.h include
> fs: add mount namespace to rbtree late
> fs: lockless mntns rbtree lookup
> rculist: add list_bidir_{del,prev}_rcu()
> fs: lockless mntns lookup for nsfs
> fs: simplify rwlock to spinlock
> seltests: move nsfs into filesystems subfolder
> selftests: add tests for mntns iteration
> selftests: remove unneeded include
> samples: add test-list-all-mounts
>
> fs/mount.h | 18 +-
> fs/namespace.c | 163 ++++++++------
> fs/nsfs.c | 5 +-
> include/linux/rculist.h | 47 +++++
> samples/vfs/.gitignore | 1 +
> samples/vfs/Makefile | 2 +-
> samples/vfs/test-list-all-mounts.c | 235 +++++++++++++++++++++
> .../selftests/{ => filesystems}/nsfs/.gitignore | 1 +
> .../selftests/{ => filesystems}/nsfs/Makefile | 4 +-
> .../selftests/{ => filesystems}/nsfs/config | 0
> .../selftests/filesystems/nsfs/iterate_mntns.c | 149 +++++++++++++
> .../selftests/{ => filesystems}/nsfs/owner.c | 0
> .../selftests/{ => filesystems}/nsfs/pidns.c | 0
> tools/testing/selftests/pidfd/pidfd.h | 1 -
> 14 files changed, 546 insertions(+), 80 deletions(-)
> ---
> base-commit: 40384c840ea1944d7c5a392e8975ed088ecf0b37
> change-id: 20241207-work-mount-rbtree-lockless-7d4071b74f18
>
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
