Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

[Mathieu Desnoyers]

----- On Jul 28, 2021, at 2:32 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote:

> On Wed, Jul 28, 2021 at 11:12 AM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>>
>> Are the percpu atomics you have in mind different from what is found in
>> Documentation/core-api/this_cpu_ops.rst ?
>>
>> Namely this_cpu_add_return(pcp, val) in this case.
> 
> Nope.
> 
> Those are only "CPU-atomic", ie atomic wrt interrupts etc.
> 
> The RCU code wants SMP-atomic, and it's mainly that we *could* do the
> addressing more efficiently.

OK, so combining the addressing tricks of this_cpu operations with
smp-atomic operations (e.g. LOCK prefix on x86). It may indeed become
worthwhile given enough users, and fast enough atomic operations.

Thanks,

Mathieu

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