linux-kernel - Re: csky: smp_mb__after

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Date:   Fri, 7 Aug 2020 08:22:36 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ren Guo <ren_guo@...ky.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org, mathieu.desnoyers@...icios.com,
        Will Deacon <will@...nel.org>
Subject: Re: csky: smp_mb__after_spinlock

Hi Peter,

On Thu, Aug 6, 2020 at 3:53 AM <peterz@...radead.org> wrote:
>
> Hi,
>
> While doing an audit of smp_mb__after_spinlock, I found that csky
> defines it, why?
>
> CSKY only has smp_mb(), it doesn't override __atomic_acquire_fence or
> otherwise special cases it's atomic*_acquire() primitives. It has an
> explicit smp_mb() in its arch_spin_lock().

Yes, csky didn't implement ACQUIRE/RELEASE in spinlock.h. So it is a
redundant and side-effect wrong macro, we should remove it to fixup.

TODO:
 - implement csky's ACQUIRE/RELEASE barrier

> Also, why have two implementations of all the locking?

I just kept my baby's codes :P. Now, we could remove it and just keep
the ticket's one.

BTW, I want to change the spinlock to qspinlock, but csky only has
32-bit atomic operation in hardware.

Any plan to deal with this in spinlock?

Maybe for the embedded scenario, qspinlock seems a bit heavy, are any
tickets-like comm spinlock infrastructures in the plan?

--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/