On Fri, Aug 15, 2025 at 9:18 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> migrate_disable() is called to disable migration in the kernel, and it is
> used togather with rcu_read_lock() oftenly.
>
> However, with PREEMPT_RCU disabled, it's unnecessary, as rcu_read_lock()
> will disable preemption, which will also disable migration.
>
> Introduce rcu_migrate_enable() and rcu_migrate_disable(), which will do
> the migration enable and disable only when the rcu_read_lock() can't do
> it.
>
> Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> ---
> include/linux/rcupdate.h | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 120536f4c6eb..0d9dbd90d025 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -72,6 +72,16 @@ static inline bool same_state_synchronize_rcu(unsigned long oldstate1, unsigned
> void __rcu_read_lock(void);
> void __rcu_read_unlock(void);
>
> +static inline void rcu_migrate_enable(void)
> +{
> + migrate_enable();
> +}
Interesting idea.
I think it has to be combined with rcu_read_lock(), since this api
makes sense only when used together.
rcu_read_lock_dont_migrate() ?
It will do rcu_read_lock() + migrate_disalbe() in PREEMPT_RCU
and rcu_read_lock() + preempt_disable() otherwise?
Also I'm not sure we can rely on rcu_read_lock()
disabling preemption in all !PREEMPT_RCU cases.
iirc it's more nuanced than that.
