6.18-stable review patch. If anyone has any objections, please let me know.
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From: Frederic Weisbecker frederic@kernel.org
[ Upstream commit fa9620355d4192200f15cb3d97c6eb9c02442249 ]
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 Stable-dep-of: 5eb579dfd46b ("timers/migration: Fix imbalanced NUMA trees") Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/time/timer_migration.c | 10 ---------- 1 file changed, 10 deletions(-)
diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c index 1e371f1fdc86c..5f8aef94ca0f7 100644 --- a/kernel/time/timer_migration.c +++ b/kernel/time/timer_migration.c @@ -1573,9 +1573,6 @@ static void tmigr_connect_child_parent(struct tmigr_group *child, { struct tmigr_walk data;
- raw_spin_lock_irq(&child->lock); - raw_spin_lock_nested(&parent->lock, SINGLE_DEPTH_NESTING); - if (activate) { /* * @child is the old top and @parent the new one. In this @@ -1596,9 +1593,6 @@ static void tmigr_connect_child_parent(struct tmigr_group *child, */ smp_store_release(&child->parent, parent);
- raw_spin_unlock(&parent->lock); - raw_spin_unlock_irq(&child->lock); - trace_tmigr_connect_child_parent(child);
if (!activate) @@ -1695,13 +1689,9 @@ static int tmigr_setup_groups(unsigned int cpu, unsigned int node) if (i == 0) { struct tmigr_cpu *tmc = per_cpu_ptr(&tmigr_cpu, cpu);
- raw_spin_lock_irq(&group->lock); - tmc->tmgroup = group; tmc->groupmask = BIT(group->num_children++);
- raw_spin_unlock_irq(&group->lock); - trace_tmigr_connect_cpu_parent(tmc);
/* There are no children that need to be connected */