Re: [RFC PATCH 1/1] hpref: Hazard Pointers with Reference Counter

[Lai Jiangshan <jiangshanlai@gmail.com>]

> +/*
> + * hpref_hp_get: Obtain a reference to a stable object, protected either
> + *               by hazard pointer (fast-path) or using reference
> + *               counter as fall-back.
> + */
> +static inline
> +bool hpref_hp_get(struct hpref_node **node_p, struct hpref_ctx *ctx)
> +{
> +       int cpu = rseq_current_cpu_raw();
> +       struct hpref_percpu_slots *cpu_slots = rseq_percpu_ptr(hpref_percpu_slots, cpu);
> +       struct hpref_slot *slot = &cpu_slots->slots[cpu_slots->current_slot];
> +       bool use_refcount = false;
> +       struct hpref_node *node, *node2;
> +       unsigned int next_slot;
> +
> +retry:
> +       node = uatomic_load(node_p, CMM_RELAXED);
> +       if (!node)
> +               return false;
> +       /* Use rseq to try setting current slot hp. Store B. */
> +       if (rseq_load_cbne_store__ptr(RSEQ_MO_RELAXED, RSEQ_PERCPU_CPU_ID,
> +                               (intptr_t *) &slot->node, (intptr_t) NULL,
> +                               (intptr_t) node, cpu)) {
> +               slot = &cpu_slots->slots[HPREF_EMERGENCY_SLOT];

Can @cpu be possibly changed? if it can, it seems @cpu and @cpu_slots
should be updated first.

> +               use_refcount = true;
> +               /*
> +                * This may busy-wait for another reader using the
> +                * emergency slot to transition to refcount.
> +                */
> +               caa_cpu_relax();
> +               goto retry;
> +       }