linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Message-ID: <1437171254.28088.87.camel@kernel.crashing.org>
Date:	Sat, 18 Jul 2015 08:14:14 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Fri, 2015-07-17 at 10:32 +0100, Will Deacon wrote:
>  static inline void arch_spin_unlock(arch_spinlock_t *lock)
>  {
> -       SYNC_IO;
> -       __asm__ __volatile__("# arch_spin_unlock\n\t"
> -                               PPC_RELEASE_BARRIER: : :"memory");
> +       smp_mb();
>         lock->slock = 0;
>  }

That probably needs to be mb() in case somebody has the expectation that
it does a barrier vs. DMA on UP.

Cheers,
Ben.


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