fs: lockless mntns lookup
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
To: Josef Bacik <josef@toxicpanda.com>
To: Jeff Layton <jlayton@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Peter Ziljstra <peterz@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
---
Changes in v4:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v3: https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-0-6e3cdaf9b280@kernel.org
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@kernel.org
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@kernel.org
--- b4-submit-tracking ---
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