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
