Re: [PATCH bpf-next 1/7] rcu: add rcu_migrate_enable and rcu_migrate_disable

[Paul E. McKenney]

On Fri, Aug 15, 2025 at 04:02:14PM +0300, Alexei Starovoitov wrote:
> 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?

That could easily be provided.  Or just make one, and if it starts
having enough use cases, it could be pulled into RCU proper.

> 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.

For once, something about RCU is non-nuanced.  But don't worry, it won't
happen again.  ;-)

In all !PREEMPT_RCU, preemption must be disabled across all RCU read-side
critical sections in order for RCU to work correctly.

							Thanx, Paul