Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation

[Paul E. McKenney]

On Wed, Aug 26, 2026 at 07:01:24PM +0100, David Woodhouse wrote:
> On 26 August 2026 17:28:27 BST, "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> >On Wed, Aug 26, 2026 at 08:05:06AM -0700, Paul E. McKenney wrote:
> >> On Wed, Aug 26, 2026 at 08:38:27AM +0100, David Woodhouse wrote:
> >> > On Wed, 2026-08-26 at 09:32 +0200, Sebastian Andrzej Siewior wrote:
> >> > > On 2026-08-25 13:51:14 [-0700], Paul E. McKenney wrote:
> >> > > > Very good, preemption disabling it is!  It is quite possible that the
> >> > > > PREEMPT_RT guys will need something else, but one thing at a time.
> >> > > 
> >> > > This reminds me of classic RCU ;) It might work in this use case but I
> >> > > am afraid that other users might come along where it actually hurts.
> >> > > With PREEMPT LAZY the preemption within the read-section should
> >> > > hopefully be the exception.
> >> > 
> >> > Given the way that PREEMPT_RT treats other locks like spinlocks and
> >> > rwlocks — turning them into sleeping locks — I suspect it makes most
> >> > sense, and is consistent, to also turn rcu_read_lock_atomic() and
> >> > synchronize_rcu_atomic() into plain rcu_read_lock() and
> >> > try_synchronize_srcu()+synchronize_scru_expedited().
> >> 
> >> That makes a lot of sense to me.
> >
> >Another option is to instead fall back to non-atomic SRCU.
> 
> That's exactly what I thought I was suggesting; I think I left an 's' out though :)

Heh!!!

About 20 years ago, I was shocked to learn of a valid use case involving
sleeping RCU readers, which resulted in SRCU.  And now it is time
to be shocked at non-sleeping sleeping RCU readers, and especially a
non-sleeping grace-period wait.  ;-)

Anyway, easy enough:

#ifdef CONFIG_PREEMPT_RT
DEFINE_SRCU(whatever);
#define my_srcu_read_lock() srcu_read_lock(&whatever);
#define my_srcu_read_unlock(idx) srcu_read_unlock(&whatever, (idx));
#define my_synchronize_srcu() synchronize_srcu(&whatever);
#else
DEFINE_SRCU_ATOMIC(whatever);
#define my_srcu_read_lock() srcu_read_lock_atomic(&whatever);
#define my_srcu_read_unlock(idx) srcu_read_unlock_atomic(&whatever, (idx));
#define my_synchronize_srcu() synchronize_srcu_atomic(&whatever);
#endif

Or is there a better way?

							Thanx, Paul