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From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: Paul Burton <paul.burton@imgtec.com>,
	linux-mips@vger.kernel.org,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/6] MIPS: Octeon: Implement __smp_store_release()
Date: Thu, 28 Jan 2021 16:04:39 +0100	[thread overview]
Message-ID: <YBLShyGWOXYVZCH8@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <3c0165d9-1814-df1d-7ec9-bf515a3996b3@nokia.com>

On Thu, Jan 28, 2021 at 01:09:39PM +0100, Alexander Sverdlin wrote:
> On 28/01/2021 12:33, Peter Zijlstra wrote:
> > On Thu, Jan 28, 2021 at 08:27:29AM +0100, Alexander Sverdlin wrote:
> > 
> >>>> +#define __smp_store_release(p, v)					\
> >>>> +do {									\
> >>>> +	compiletime_assert_atomic_type(*p);				\
> >>>> +	__smp_wmb();							\
> >>>> +	__smp_rmb();							\
> >>>> +	WRITE_ONCE(*p, v);						\
> >>>> +} while (0)

> I actually hoped you will remember the discussion you've participated 5 years
> ago and (in my understanding) actually already agreed that the solution itself
> is not broken:
> 
> https://lore.kernel.org/lkml/20151112180003.GE17308@twins.programming.kicks-ass.net/

My memory really isn't that good. I can barely remember what I did 5
weeks ago, 5 years ago might as well have never happened.

> Could you please just suggest the proper comment you expect to be added here,
> because there is no doubts, you have much more experience here than me?

So for store_release I'm not too worried, and provided no read
speculation, wmb is indeed sufficient. This is because our store_release
is RCpc.

Something like:

/*
 * Because Octeon does not do read speculation, an smp_wmb()
 * is sufficient to ensure {load,store}->{store} order.
 */
#define __smp_store_release(p, v) \
do { \
	compiletime_assert_atomic_type(*p); \
	__smp_wmb(); \
	WRITE_ONCE(*p, v); \
} while (0)

  reply	other threads:[~2021-01-28 15:06 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-27 20:36 [PATCH 0/6] MIPS: qspinlock: Try to reduce reduce the spinlock regression Alexander A Sverdlin
2021-01-27 20:36 ` [PATCH 1/6] MIPS: Octeon: Implement __smp_store_release() Alexander A Sverdlin
2021-01-27 22:32   ` Peter Zijlstra
2021-01-28  7:27     ` Alexander Sverdlin
2021-01-28 11:33       ` Peter Zijlstra
2021-01-28 11:52         ` Alexander Sverdlin
2021-01-28 14:57           ` Peter Zijlstra
2021-01-28 15:15             ` Peter Zijlstra
2021-01-28 12:09         ` Alexander Sverdlin
2021-01-28 15:04           ` Peter Zijlstra [this message]
2021-01-27 20:36 ` [PATCH 2/6] MIPS: Implement atomic_cmpxchg_relaxed() Alexander A Sverdlin
2021-01-27 20:36 ` [PATCH 3/6] MIPS: Octeon: qspinlock: Flush write buffer Alexander A Sverdlin
2021-01-27 22:34   ` Peter Zijlstra
2021-01-28  7:29     ` Alexander Sverdlin
2021-01-28 11:35       ` Peter Zijlstra
2021-01-28 12:13         ` Alexander Sverdlin
2021-01-28 15:26           ` Peter Zijlstra
2021-01-27 20:36 ` [PATCH 4/6] MIPS: Octeon: qspinlock: Exclude mmiowb() Alexander A Sverdlin
2021-01-27 22:35   ` Peter Zijlstra
2021-01-27 20:36 ` [PATCH 5/6] MIPS: Provide {atomic_}xchg_relaxed() Alexander A Sverdlin
2021-01-27 20:36 ` [PATCH 6/6] MIPS: cmpxchg: Use cmpxchg_local() for {cmp_}xchg_small() Alexander A Sverdlin
2021-01-27 22:37   ` Peter Zijlstra

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