Re: [PATCH 4/8] shazptr: Avoid synchronize_shaptr() busy waiting

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Jun 25, 2025 at 03:56:05PM +0200, Frederic Weisbecker wrote:
> Le Tue, Jun 24, 2025 at 08:10:57PM -0700, Boqun Feng a écrit :
> > +static void synchronize_shazptr_normal(void *ptr)
> > +{
> > +	int cpu;
> > +	unsigned long blocking_grp_mask = 0;
> > +
> > +	smp_mb(); /* Synchronize with the smp_mb() in shazptr_acquire(). */
> > +
> > +	for_each_possible_cpu(cpu) {
> > +		void **slot = per_cpu_ptr(&shazptr_slots, cpu);
> > +		void *val;
> > +
> > +		/* Pair with smp_store_release() in shazptr_clear(). */
> > +		val = smp_load_acquire(slot);
> > +
> > +		if (val == ptr || val == SHAZPTR_WILDCARD)
> > +			blocking_grp_mask |= 1UL << (cpu / shazptr_scan.cpu_grp_size);
> > +	}
> > +
> > +	/* Found blocking slots, prepare to wait. */
> > +	if (blocking_grp_mask) {
> 
> synchronize_rcu() here would be enough since all users have preemption disabled.
> But I guess this defeats the performance purpose? (If so this might need a
> comment somewhere).
> 

synchronize_shazptr_normal() cannot wait for a whole grace period,
because the point of hazard pointers is to avoid waiting for unrelated
readers.

> I guess blocking_grp_mask is to avoid allocating a cpumask (again for
> performance purpose? So I guess synchronize_shazptr_normal() has some perf

If we are talking about {k,v}malloc allocation:
synchronize_shazptr_normal() would mostly be used in cleanup/free path
similar to synchronize_rcu(), therefor I would like to avoid "allocating
memory to free memory".

> expectations?)
> 
> One possibility is to have the ptr contained in:
> 
> struct hazptr {
>        void *ptr;
>        struct cpumask scan_mask
> };
> 

You mean updaters passing a `struct hazptr *` into
synchronize_shazptr_normal()? That may be a good idea, if multiple
updaters can share the same `struct hazptr *`, we can add that later,
but...

> And then the caller could simply scan itself those remaining CPUs without
> relying on the kthread.

.. this is a bad idea, sure, we can always burn some CPU time to scan,
but local optimization doesn't mean global optimization, if in the
future, we have a lots of synchronize_shazptr_normal()s happening at
the same time, the self busy-waiting scan would become problematic.

Regards,
Boqun

> 
> But I'm sure there are good reasons for now doing that :-)
> 
> Thanks.
> 
> -- 
> Frederic Weisbecker
> SUSE Labs