Re: [PATCH hazptr 4/4] hazptr: Introduce "try acquire" fast path, fallback to overflow list

[Boqun Feng <boqun@kernel.org>]

On Mon, Sep 28, 2026 at 12:32:34PM +0100, Gary Guo wrote:
[...]
> >> For some context, the current Rust revocable implementation uses RCU, but this
> >> is limiting the case where it can be used. The current C revocable series in
> >> https://lore.kernel.org/all/20260912123529.7951-1-tzungbi@kernel.org/ uses SRCU.
> >> 
> >> I think this use case is a good one for hazptr, in fact, I have encouraged Alvin
> >> Sun to try it out and you can see an implementation (Rust) in here:
> >> https://lore.kernel.org/rust-for-linux/20260326-b4-tyr-debugfs-v1-6-074badd18716@linux.dev/
> >> Although, over the course of the year, we have been reducing the amount of
> >> revocable usage and shifting to represent things with lifetime..
> >> 
> >
> > Paul asked me for the Rust usage a few days ago, and since we moved to
> > a lifetime based approach [1], which is better IMO, I don't think
> > switching to hazptr will should observable difference here, especially
> > for a real workload improvement. It's still worth trying to see how
> > hazptr in Rust would work for this, but it's less a sufficient condition
> > to merge hazptr from my understanding. Of course I could be wrong.
> >
> > [1]: https://lore.kernel.org/rust-for-linux/20260517000149.3226762-1-dakr@kernel.org/
> 
> What we get rid of is to use `Revocable` (or `DevRes`) to protect resources
> where we _know_ that the resource is alive (for example, in device private data,
> because it's teared down by driver-core on unbind).
> 
> However, there are cases where we can still have the `Revocable` pattern, for
> example when a class device is exposed to userspace and the bus device is
> unbinding. This is a pretty recurring pattern too, and cannot be handled via
> lifetime alone. This is what motivates the C revocable series.
> 

Not trying to discourage switching to hazptr in `Revocable`, but I think
the only shortcoming of RCU there is "unable to sleep while referencing
a `Revocable`", right? Certainly hazptr can help on that but so does
SRCU. I'm unsure hazptr can show observable advantages against SRCU
here. But I could be wrong.

Regards,
Boqun

> Best,
> Gary