On Sun, Aug 17, 2025 at 10:01:23AM +0800, Menglong Dong wrote:
> On Fri, Aug 15, 2025 at 11:31 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > 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.
>
> Hi, do you mean that we should start with a single
> use case? In this series, I started it with the BPF
> subsystem. Most of the situations are similar, which will
> call rcu_read_lock+migrate_disable and run bpf prog.
Other than my wanting more compact code, what you did in your patch
series is fine.
Thanx, Paul
> > > 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.
>
> Great! I worried about this part too.
>
> Thanks!
> Menglong Dong
>
> >
> > Thanx, Paul
