timers/migration: Remove locking on group connection

timers/migration: Remove locking on group connection

Initializing the tmc's group, the group's number of children and the
group's parent can all be done without locking because:

  1) Reading the group's parent and its group mask is done locklessly.

  2) The connections prepared for a given CPU hierarchy are visible to the
     target CPU once online, thanks to the CPU hotplug enforced memory
     ordering.

  3) In case of a newly created upper level, the new root and its
     connections and initialization are made visible by the CPU which made
     the connections. When that CPUs goes idle in the future, the new link
     is published by tmigr_inactive_up() through the atomic RmW on
     ->migr_state.

  4) If CPUs were still walking up the active hierarchy, they could observe
     the new root earlier. In this case the ordering is enforced by an
     early initialization of the group mask and by barriers that maintain
     address dependency as explained in:

     b729cc1ec21a ("timers/migration: Fix another race between hotplug and idle entry/exit")
     de3ced72a792 ("timers/migration: Enforce group initialization visibility to tree walkers")

  5) Timers are propagated by a chain of group locking from the bottom to
     the top. And while doing so, the tree also propagates groups links
     and initialization. Therefore remote expiration, which also relies
     on group locking, will observe those links and initialization while
     holding the root lock before walking the tree remotely and update
     remote timers. This is especially important for migrators in the
     active hierarchy that may observe the new root early.

Therefore the locking is unnecessary at initialization. If anything, it
just brings confusion. Remove it.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251024132536.39841-3-frederic@kernel.org