On Sat, Oct 05, 2024 at 06:04:44PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 04, 2024 at 02:27:33PM -0400, Mathieu Desnoyers wrote:
> > +void hp_scan(struct hp_slot __percpu *percpu_slots, void *addr,
> > + void (*retire_cb)(int cpu, struct hp_slot *slot, void *addr))
> > +{
> > + int cpu;
> > +
> > + /*
> > + * Store A precedes hp_scan(): it unpublishes addr (sets it to
> > + * NULL or to a different value), and thus hides it from hazard
> > + * pointer readers.
> > + */
This should probably assert we're in a preemptible context. Otherwise
people will start using this in non-preemptible context and then we get
to unfuck things later.
> > +
> > + if (!addr)
> > + return;
> > + /* Memory ordering: Store A before Load B. */
> > + smp_mb();
> > + /* Scan all CPUs slots. */
> > + for_each_possible_cpu(cpu) {
> > + struct hp_slot *slot = per_cpu_ptr(percpu_slots, cpu);
> > +
> > + if (retire_cb && smp_load_acquire(&slot->addr) == addr) /* Load B */
> > + retire_cb(cpu, slot, addr);
>
> Is retirce_cb allowed to cmpxchg the thing?
>
> > + /* Busy-wait if node is found. */
> > + while ((smp_load_acquire(&slot->addr)) == addr) /* Load B */
> > + cpu_relax();
>
> This really should be using smp_cond_load_acquire()
>
> > + }
> > +}
