[PATCH v8 0/6] kernfs: Use RCU to access kernfs_node::{parent|name}.

Sebastian Andrzej Siewior
2025-02-13T14:50:28Z

Hi,

This started as a bug report by Hillf Danton and aims to access
kernfs_node::{name|parent} with RCU to avoid the lock during
kernfs_path_from_node().

I've split the individual fixes in separate patches (#1 to #4). I've
also split the ::parent and ::name RCU conversation into a single patch
(#5 and #6).

v7…v8 https://lore.kernel.org/all/20250203135023.416828-1-bigeasy@linutronix.de/
- A user in selftests/bpf is fixed in #5 as part of the rename.

v6…v7 https://lore.kernel.org/all/20250130140207.1914339-1-bigeasy@linutronix.de/
- Rebase on v6.14-rc1

v5…v6 https://lore.kernel.org/all/20250128084226.1499291-1-bigeasy@linutronix.de/
- s/rdt_kn_get_name/rdt_kn_name/
- s/rdt_get_kn_parent_priv/rdt_kn_parent_priv/
- s/kn_get_priv/kn_priv/
- The comment, that has been removed in kernfs_put(), is back.
- Using rcu_access_pointer() in kernfs_activate_one() and kernfs_dir_pos()
instead of kernfs_parent() where the pointer is not dereferenced but
just compared.

v4…v5 https://lore.kernel.org/all/20250124174614.866884-1-bigeasy@linutronix.de/
- rdtgroup:
- Add a comment to rdt_get_kn_parent_priv() regarding lifetime of
parent.
- Move individual rcu_dereference_check() invocations into
rdt_kn_parent() with a comment on lifetime.
- Use rcu_access_pointer() in kernfs_to_rdtgroup() instead
rcu_dereference_check(, true)
- s/kernfs_rcu_get_name/kernfs_rcu_name/
- Move all rcu_dereference_check() within kernfs into kernfs_parent()
and extend its checks to have all cases in one spot. Document why
each case makes sense.
- kernfs_notify_workfn(): Do unlocks in the reverse order of locks.
- Add kernfs_root_flags() and use it in cgroup's kn_get_priv() to
check the right KERNFS_ROOT_INVARIANT_PARENT flag.

v3: https://lore.kernel.org/all/20241121175250.EJbI7VMb@linutronix.de/
v2: https://lore.kernel.org/all/20241112155713.269214-1-bigeasy@linutronix.de/
v1: https://lore.kernel.org/all/20241108222406.n5azgO98@linutronix.de/

Sebastian

Sebastian Andrzej Siewior (6):
kernfs: Acquire kernfs_rwsem in kernfs_notify_workfn().
kernfs: Acquire kernfs_rwsem in kernfs_get_parent_dentry().
kernfs: Acquire kernfs_rwsem in kernfs_node_dentry().
kernfs: Don't re-lock kernfs_root::kernfs_rwsem in
kernfs_fop_readdir().
kernfs: Use RCU to access kernfs_node::parent.
kernfs: Use RCU to access kernfs_node::name.

arch/x86/kernel/cpu/resctrl/internal.h | 5 +
arch/x86/kernel/cpu/resctrl/pseudo_lock.c | 14 +-
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 73 +++---
fs/kernfs/dir.c | 211 ++++++++++--------
fs/kernfs/file.c | 6 +-
fs/kernfs/kernfs-internal.h | 37 ++-
fs/kernfs/mount.c | 21 +-
fs/kernfs/symlink.c | 30 +--
fs/sysfs/dir.c | 2 +-
fs/sysfs/file.c | 24 +-
include/linux/kernfs.h | 14 +-
kernel/cgroup/cgroup-v1.c | 2 +-
kernel/cgroup/cgroup.c | 24 +-
security/selinux/hooks.c | 7 +-
.../selftests/bpf/progs/profiler.inc.h | 2 +-
15 files changed, 308 insertions(+), 164 deletions(-)