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

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

Posted by Sebastian Andrzej Siewior 10 months, 1 week ago

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(-)

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

Posted by Tejun Heo 10 months, 1 week ago

On Thu, Feb 13, 2025 at 03:50:17PM +0100, Sebastian Andrzej Siewior wrote:
> 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).

Greg, the whole series looks good to me. Probably ready to iterate in tree
even if there are more updates to make?

Thanks.

-- 
tejun

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

Posted by Greg Kroah-Hartman 10 months, 1 week ago

On Thu, Feb 13, 2025 at 06:33:46AM -1000, Tejun Heo wrote:
> On Thu, Feb 13, 2025 at 03:50:17PM +0100, Sebastian Andrzej Siewior wrote:
> > 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).
> 
> Greg, the whole series looks good to me. Probably ready to iterate in tree
> even if there are more updates to make?

Give me a chance to catch up with my pending patch review queue, I've
been in CRA meetings lately and am way behind...

Don't worry, it's not lost.

thanks,

greg k-h