Re: [RFC PATCH v4 3/4] hazptr: Implement Hazard Pointers

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Dec 19, 2025 at 05:39:00PM -0500, Joel Fernandes wrote:
> 
> 
> On 12/19/2025 5:19 PM, Mathieu Desnoyers wrote:
> > On 2025-12-19 10:42, Joel Fernandes wrote:
> >>
> >> IMHO the overflow case is "special" and should not happen often, otherwise
> >> things are "bad" anyway. I am not sure if this kind of complexity will be worth
> >> it unless we know HP forward-progress is a real problem. Also, since HP acquire
> >> will be short lived, are we that likely to not get past a temporary shortage of
> >> slots?
> > 
> > Given that we have context switch integration which moves the active
> > per-cpu slots to the overflow list, I can see that even not-so-special
> > users which get preempted or block regularly with active per-cpu slots
> > could theoretically end up preventing HP scan progress.
> 
> Yeah, I see. That's the 'problem' with the preemption design of this patchset.
> You always have to move it in and out of overflow list on preemption even when
> there is no overflow AFAICS. My idea (which has its own issues) does not require
> that on preemption.
> 
> > Providing HP scan progress guarantees is IMO an important aspect to
> > consider if we want to ensure the implementation does not cause subtle
> > HP scan stall when its amount of use will scale up.
> 
> Sure. But if you didn't have to deal with a list in the 'normal' case (not over
> saturated slots case), then it wouldn't be that big a deal.
> 

With PREEPT_HAZPTR, the overflow list will be used if a task gets
preempted while using hazptrs, without PREEPT_HAZPTR, the overflow list
will be used if there are more than 8 slots used on the CPU (most likely
due to task being preempted with a hazptr slot being used). Therefore,
if this hazptr design it to have a few users and we want to support that
preemptible hazard pointer read-side section, soon or later we need to
resolve this scanning racing with readers (in context switches) on
overflow list issue.

Surely we don't need to have something perfect in day 1, but planning a
bit ahead won't hurt ;-)

Regards,
Boqun

> >> Perhaps the forward-progress problem should be rephrased to the following?: If a
> >> reader hit an overflow slot, it should probably be able to get a non-overflow
> >> slot soon, even if hazard pointer slots are over-subscribed.
> > 
> > Those are two distinct forward progress guarantees, and I think both are
> > important:
> > 
> > * Forward progress of HP readers,
> > * Forward progress of HP scan.
> 
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