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

[Mathieu Desnoyers]

On 2024-10-03 02:24, Boqun Feng wrote:
> On Tue, Oct 01, 2024 at 09:02:04PM -0400, Mathieu Desnoyers wrote:
[...]
> > +/*
> > + * hp_allocate: Allocate a hazard pointer.
> > + *
> > + * Allocate a hazard pointer slot for @addr. The object existence should
> > + * be guaranteed by the caller.
> > + *
> > + * Returns a hazard pointer context.
> > + */
> > +static inline
> > +struct hp_ctx hp_allocate(struct hp_slot __percpu *percpu_slots, void *addr)
> > +{
> > +	struct hp_slot *slot;
> > +	struct hp_ctx ctx;
> > +
> > +	if (!addr)
> > +		goto fail;
> > +	slot = this_cpu_ptr(percpu_slots);
>
> Are you assuming this is called with preemption disabled? Otherwise,
> there could two threads picking up the same hazard pointer slot on one
> CPU,

Indeed, this minimalist implementation only covers the preempt-off
use-case, where there is a single use of HP per CPU at any given
time (e.g. for the lazy mm use-case). It expects to be called
from preempt-off context. I will update the comment accordingly.

> > +	/*
> > +	 * A single hazard pointer slot per CPU is available currently.
> > +	 * Other hazard pointer domains can eventually have a different
> > +	 * configuration.
> > +	 */
> > +	if (READ_ONCE(slot->addr))
> > +		goto fail;
>
> .. and they could both read an empty slot, and both think they
> successfully protect the objects, which could be different objects.
>
> Or am I missing something subtle here?

You are correct, I should document this.

> > +	WRITE_ONCE(slot->addr, addr);	/* Store B */
> > +	ctx.slot = slot;
> > +	ctx.addr = addr;
> > +	return ctx;
> > +
> > +fail:
> > +	ctx.slot = NULL;
> > +	ctx.addr = NULL;
> > +	return ctx;
> > +}
> > +
> > +/*
> > + * hp_dereference_allocate: Dereference and allocate a hazard pointer.
> > + *
> > + * Returns a hazard pointer context.
> > + */
> > +static inline
> > +struct hp_ctx hp_dereference_allocate(struct hp_slot __percpu *percpu_slots, void * const * addr_p)
> > +{
> > +	struct hp_slot *slot;
> > +	void *addr, *addr2;
> > +	struct hp_ctx ctx;
> > +
> > +	addr = READ_ONCE(*addr_p);
> > +retry:
> > +	ctx = hp_allocate(percpu_slots, addr);
> > +	if (!hp_ctx_addr(ctx))
> > +		goto fail;
> > +	/* Memory ordering: Store B before Load A. */
> > +	smp_mb();
> > +	/*
> > +	 * Use RCU dereference without lockdep checks, because
> > +	 * lockdep is not aware of HP guarantees.
> > +	 */
> > +	addr2 = rcu_access_pointer(*addr_p);	/* Load A */
>
> Why rcu_access_pointer() instead of READ_ONCE()? Because you want to
> mark the head of address dependency?

Yes, the intent here is to mark the address dependency and provide
a publication guarantee similar to RCU pairing rcu_assign_pointer
and rcu_dereference. Do you see any reason why READ_ONCE() would
suffice here ?

Thanks,

Mathieu


> Regards,
> Boqun
>
> > +	/*
> > +	 * If @addr_p content has changed since the first load,
> > +	 * clear the hazard pointer and try again.
> > +	 */
> > +	if (!ptr_eq(addr2, addr)) {
> > +		WRITE_ONCE(slot->addr, NULL);
> > +		if (!addr2)
> > +			goto fail;
> > +		addr = addr2;
> > +		goto retry;
> > +	}
> > +	ctx.slot = slot;
> > +	ctx.addr = addr2;
> > +	return ctx;
> > +
> > +fail:
> > +	ctx.slot = NULL;
> > +	ctx.addr = NULL;
> > +	return ctx;
> > +}
> > +
> [...]

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com