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

[Mathieu Desnoyers]

----- On Jul 28, 2021, at 1:58 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote:

> On Wed, Jul 28, 2021 at 10:37 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>>
>> +/*
>> + * Increment the current CPU's rcu_data structure's ->dynticks field
>> + * with ordering.  Return the new value.
>> + */
>> +static noinstr unsigned long rcu_dynticks_inc(int incby)
>> +{
>> +       return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
>> +}
> 
> Maybe inline?
> 
> But more I reacted to how we sadly don't have percpu atomics. They'd
> be fairly easy to add on x86, but I guess it's not a huge deal.

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.

I must be missing something subtle because AFAIU those are already
available. Those per-cpu atomics don't provide any memory ordering
though, which may be why those are not used here.

Thanks,

Mathieu

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