Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

[Peter Zijlstra]

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()
> 
> > +	}
> > +}