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

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).

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

One possibility is to have the ptr contained in:

struct hazptr {
       void *ptr;
       struct cpumask scan_mask
};

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

But I'm sure there are good reasons for now doing that :-)

Thanks.

-- 
Frederic Weisbecker
SUSE Labs