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Date:   Wed, 28 Jul 2021 14:12:05 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and
 updates
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Thread-Topic: Weaken ->dynticks accesses and updates
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----- On Jul 28, 2021, at 1:58 PM, Linus Torvalds torvalds@linux-foundation.org wrote:

> On Wed, Jul 28, 2021 at 10:37 AM Paul E. McKenney <paulmck@kernel.org> 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