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

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Message-ID: <878sg07twt.fsf@mpe.ellerman.id.au>
Date:   Fri, 03 Jul 2020 20:52:02 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>, 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

Nicholas Piggin <npiggin@...il.com> writes:
> Excerpts from Will Deacon's message of July 2, 2020 8:35 pm:
>> On Thu, Jul 02, 2020 at 08:25:43PM +1000, Nicholas Piggin wrote:
>>> 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.
>> 
>> Is this still an issue, though?
>> 
>> See 38b850a73034 (where we added a similar barrier on arm64) and then
>> c6f5d02b6a0f (where we removed it).
>> 
>
> Oh nice, I didn't know that went away. Thanks for the heads up.

Argh! I spent so much time chasing that damn bug in the ipc code.

> I'm going to say I'm too scared to remove it while changing the
> spinlock algorithm, but I'll open an issue and we should look at 
> removing it.

Sounds good.

cheers