Re: [PATCH 2/2] PM: cpu: Restore synchronize_rcu() to cpu_pm_unregister_notifier()

[Rafael J. Wysocki (Intel)]

On Thu, Aug 13, 2026 at 9:43 AM Bradley Morgan <include@xxxxxxxxx> wrote:
>
> 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: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Bradley Morgan <include@xxxxxxxxx>

Acked-by: Rafael J. Wysocki (Intel) <rafael@xxxxxxxxxx>

or if you want me to pick up this series, I need an ACK on the first patch.

> ---
>  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 <linux/cpu_pm.h>
>  #include <linux/module.h>
>  #include <linux/notifier.h>
> +#include <linux/rcupdate.h>
>  #include <linux/spinlock.h>
>  #include <linux/syscore_ops.h>
>
> @@ -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);
> --
> 2.47.3
>