linux-kernel - Re: [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks

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Date:   Thu, 02 Jul 2020 20:25:43 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Will Deacon <will@...nel.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, Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks

Excerpts from Will Deacon's message of July 2, 2020 6:02 pm:
> On Thu, Jul 02, 2020 at 05:48:36PM +1000, Nicholas Piggin wrote:
>> diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h
>> new file mode 100644
>> index 000000000000..f84da77b6bb7
>> --- /dev/null
>> +++ b/arch/powerpc/include/asm/qspinlock.h
>> @@ -0,0 +1,20 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef _ASM_POWERPC_QSPINLOCK_H
>> +#define _ASM_POWERPC_QSPINLOCK_H
>> +
>> +#include <asm-generic/qspinlock_types.h>
>> +
>> +#define _Q_PENDING_LOOPS	(1 << 9) /* not tuned */
>> +
>> +#define smp_mb__after_spinlock()   smp_mb()
>> +
>> +static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
>> +{
>> +	smp_mb();
>> +	return atomic_read(&lock->val);
>> +}
> 
> Why do you need the smp_mb() here?

A long and sad tale that ends here 51d7d5205d338

Should probably at least refer to that commit from here, since this one 
is not going to git blame back there. I'll add something.

Thanks,
Nick