linux-kernel - Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

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Date:   Fri, 24 Jul 2020 02:12:21 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Anton Blanchard <anton@...abs.org>,
        Boqun Feng <boqun.feng@...il.com>, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

Excerpts from Waiman Long's message of July 24, 2020 12:29 am:
> On 7/23/20 9:30 AM, Nicholas Piggin wrote:
>>> I would prefer to extract out the pending bit handling code out into a
>>> separate helper function which can be overridden by the arch code
>>> instead of breaking the slowpath into 2 pieces.
>> You mean have the arch provide a queued_spin_lock_slowpath_pending
>> function that the slow path calls?
>>
>> I would actually prefer the pending handling can be made inline in
>> the queued_spin_lock function, especially with out-of-line locks it
>> makes sense to put it there.
>>
>> We could ifdef out queued_spin_lock_slowpath_queue if it's not used,
>> then __queued_spin_lock_slowpath_queue would be inlined into the
>> caller so there would be no split?
> 
> The pending code is an optimization for lightly contended locks. That is 
> why I think it is appropriate to extract it into a helper function and 
> mark it as such.
> 
> You can certainly put the code in the arch's spin_lock code, you just 
> has to override the generic pending code by a null function.

I see what you mean. I guess that would work fine.

Thanks,
Nick