Re: [PATCH hazptr v3 4/4] hazptr: Introduce "try acquire" fast path, fallback to overflow list

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-10-11 15:59, Bradley Morgan wrote:
> On 11 October 2026 20:48:50 BST, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>> Introduce a "try acquire" hazard pointer fast path, which performs an
>> early load of the address to store it into the hazard pointer slot, and
>> then re-loads that address after a barrier to check whether it has
>> changed meanwhile.
>>
>> On comparison failure, rather than re-try, guarantee forward progress by
>> falling back to the __hazptr_acquire slow path on failure.
>>
>> The acquire slow path attempts a try-acquire for any available per-CPU
>> slot. If that fails, it chains the backup slot into the overflow list,
>> therefore guaranteeing forward progress for both hazard pointer
>> read-side and synchronize:
>>
>> - Readers set the wildcard, and then proceed to set the more
>>   specific address to replace the wildcard.
>>
>> - One synchronize alternates between two overflow list periods,
>>   scanning each one while readers are added to the other period,
>>   thus preventing a steady flow of readers from preventing
>>   synchronize forward progress.
>>
>> With this change, the scan on per-CPU slots don't need to expect a
>> wildcard anymore, because none can be produced by readers. Wildcards are
>> only expected within overflow lists.
>>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Suggested-by: Gary Guo <gary@garyguo.net>
>> Link: https://lore.kernel.org/lkmm/DLK9MW1N86KD.2GRWUAD2NX4FU@garyguo.net/
>> Reviewed-by: Gary Guo <gary@garyguo.net>
> 
> Hi, my R-B still stands from
> 
> https://lore.kernel.org/all/C53DEBF5-555F-44A8-A11C-A15C83AAD341@mainlining.org/
> 
> Reviewed-by: Bradley Morgan <brads@mainlining.org>

Sorry, I forgot to add it to my commit message. I'll add it to the next
round (if there is one), else hopefully Paul can add the R-B tag if he
picks up this series.

Thanks!

Mathieu


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