Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

[Mathieu Desnoyers]

On 2024-09-17 16:33, Boqun Feng wrote:
[...]
> The synchronization between readers and updaters is built around "hazard
> pointer slots": a slot readers can use to store a pointer value.
>
> Reader side protection:
>
> 1.	Read the value of a pointer to the target data element.
> 2.	Store it to a hazard pointer slot.
> 3.	Enforce full ordering (e.g. smp_mb()).
> 4.	Re-read the original pointer, reset the slot and retry if the
> 	value changed.
> 5.	Otherwise, the continued existence of the target data element
> 	is guaranteed.
>
> Updater side check:
>
> 1.	Unpublish the target data element (e.g. setting the pointer
> 	value to NULL).
> 2.	Enforce full ordering.
> 3.	Read the value from a hazard pointer slot.
> 4.	If the value doesn't match the target data element, then this
> 	slot doesn't represent a reference to it.
> 5.	Otherwise, updater needs to re-check (step 3).

Cool! I look forward to see where this is meant to be used. I would
expect it to be a useful tool to speed up reference counting of
things like the mm_struct and for TLB flush IPIs.

On a related note, with a userspace port in mind, the membarrier(2)
syscall can be useful to turn the smp_mb() in (3) from the reader
side into a simple compiler barrier, assuming (2) from the updater
is using membarrier. If IPIs are acceptable (or already required) for
some kernel use-cases, this means a similar asymmetric fence scheme
could be used to speed up readers.

Thanks,

Mathieu

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