Centos-kernel-stream-9

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David Arcari e18c58be0a kernfs: Use RCU to access kernfs_node::name.

JIRA: https://issues.redhat.com/browse/RHEL-118965

commit 741c10b096bc4dd79cd9f215b6ef173bb953e75c
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Thu Feb 13 15:50:23 2025 +0100

    kernfs: Use RCU to access kernfs_node::name.

    Using RCU lifetime rules to access kernfs_node::name can avoid the
    trouble with kernfs_rename_lock in kernfs_name() and kernfs_path_from_node()
    if the fs was created with KERNFS_ROOT_INVARIANT_PARENT. This is usefull
    as it allows to implement kernfs_path_from_node() only with RCU
    protection and avoiding kernfs_rename_lock. The lock is only required if
    the __parent node can be changed and the function requires an unchanged
    hierarchy while it iterates from the node to its parent.
    The change is needed to allow the lookup of the node's path
    (kernfs_path_from_node()) from context which runs always with disabled
    preemption and or interrutps even on PREEMPT_RT. The problem is that
    kernfs_rename_lock becomes a sleeping lock on PREEMPT_RT.

    I went through all ::name users and added the required access for the lookup
    with a few extensions:
    - rdtgroup_pseudo_lock_create() drops all locks and then uses the name
      later on. resctrl supports rename with different parents. Here I made
      a temporal copy of the name while it is used outside of the lock.

    - kernfs_rename_ns() accepts NULL as new_parent. This simplifies
      sysfs_move_dir_ns() where it can set NULL in order to reuse the current
      name.

    - kernfs_rename_ns() is only using kernfs_rename_lock if the parents are
      different. All users use either kernfs_rwsem (for stable path view) or
      just RCU for the lookup. The ::name uses always RCU free.

    Use RCU lifetime guarantees to access kernfs_node::name.

    Suggested-by: Tejun Heo <tj@kernel.org>
    Acked-by: Tejun Heo <tj@kernel.org>
    Reported-by: syzbot+6ea37e2e6ffccf41a7e6@syzkaller.appspotmail.com
    Closes: https://lore.kernel.org/lkml/67251dc6.050a0220.529b6.015e.GAE@google.com/
    Reported-by: Hillf Danton <hdanton@sina.com>
    Closes: https://lore.kernel.org/20241102001224.2789-1-hdanton@sina.com
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Link: https://lore.kernel.org/r/20250213145023.2820193-7-bigeasy@linutronix.de
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Conflicts:
- fs/kernfs/file.c: 1 hunk modified

Assisted-by: Patchpal AI
Signed-off-by: David Arcari <darcari@redhat.com>