From 61f7eee7d4664079fbce821576891c3ee5ccf597 Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Thu, 13 Aug 2026 07:43:43 +0000 Subject: [PATCH] PM: CPU: Restore synchronize_rcu() to cpu_pm_unregister_notifier() cpu_pm_notify() walks the notifier chain lockless under only rcu_read_lock(). That only works if a removed notifier block is not freed until every concurrent walker is done with it. The chain used to be an atomic_notifier, whose unregister ends in synchronize_rcu(). Commit b2f6662ac08d ("PM: cpu: Make notifier chain use a raw_spinlock_t") switched it over to a raw_notifier, and in doing so replaced that with raw_notifier_chain_unregister(), which does not synchronize. The grace period quietly went away, so a driver that frees the memory holding its notifier block right after cpu_pm_unregister_notifier() returns can race with a concurrent walker: cpu1 (idle exit) cpu2 (driver remove) ---------------- -------------------- cpu_pm_notify(CPU_PM_EXIT) rcu_read_lock() nb = rcu_dereference_raw(*nl) cpu_pm_unregister_notifier(&od->nb) unlink, no grace period remove() frees od (devm) nb->notifier_call(nb, ...) /* use after free */ The rcu_read_lock() on cpu1 does not stop cpu2 from freeing the block, and nothing else does. The read side is the idle exit path, so this can hit on any system where a driver with a cpu_pm notifier gets unbound. Add the grace period back. It is only needed when a notifier was actually removed, so wait on success and return -ENOENT without waiting otherwise. The kerneldoc gets back the "may sleep" note that the same commit dropped. Fixes: b2f6662ac08d ("PM: cpu: Make notifier chain use a raw_spinlock_t") Cc: All applicable Signed-off-by: Bradley Morgan [ rjw: Subject tweak ] Link: https://patch.msgid.link/20260813074343.26460-3-include@grrlz.net Signed-off-by: Rafael J. Wysocki --- kernel/cpu_pm.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/kernel/cpu_pm.c b/kernel/cpu_pm.c index 7481fbb947d3..a2a598ad6e6d 100644 --- a/kernel/cpu_pm.c +++ b/kernel/cpu_pm.c @@ -10,6 +10,7 @@ #include #include #include +#include #include #include @@ -76,7 +77,8 @@ EXPORT_SYMBOL_GPL(cpu_pm_register_notifier); * * Remove a driver from the CPU PM notifier list. * - * This function has the same return conditions as raw_notifier_chain_unregister. + * This function may sleep, and has the same return conditions as + * raw_notifier_chain_unregister. */ int cpu_pm_unregister_notifier(struct notifier_block *nb) { @@ -86,6 +88,11 @@ int cpu_pm_unregister_notifier(struct notifier_block *nb) raw_spin_lock_irqsave(&cpu_pm_notifier.lock, flags); ret = raw_notifier_chain_unregister(&cpu_pm_notifier.chain, nb); raw_spin_unlock_irqrestore(&cpu_pm_notifier.lock, flags); + + /* Wait for the rcu_read_lock() walkers in cpu_pm_notify(). */ + if (!ret) + synchronize_rcu(); + return ret; } EXPORT_SYMBOL_GPL(cpu_pm_unregister_notifier);