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

[Mathieu Desnoyers]

On 2026-10-11 15:59, Bradley Morgan wrote:
> On 11 October 2026 20:48:50 BST, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > Suggested-by: Gary Guo <gary@xxxxxxxxxxx>
> > Link: https://lore.kernel.org/lkmm/DLK9MW1N86KD.2GRWUAD2NX4FU@xxxxxxxxxxx/
> > Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
>
> Hi, my R-B still stands from
>
> https://lore.kernel.org/all/C53DEBF5-555F-44A8-A11C-A15C83AAD341@xxxxxxxxxxxxxx/
>
> Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>

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