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Date:   Fri, 18 Jun 2021 13:26:32 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        Will Deacon <will.deacon@arm.com>,
        "Russell King, ARM Linux" <linux@armlinux.org.uk>,
        David Sehr <sehr@google.com>,
        Paul Mackerras <paulus@samba.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        linux-arch <linux-arch@vger.kernel.org>, x86 <x86@kernel.org>,
        Andrew Hunter <ahh@google.com>,
        Greg Hackmann <ghackmann@google.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Paul <paulmck@linux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Avi Kivity <avi@scylladb.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
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        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
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References: <20180129202020.8515-1-mathieu.desnoyers@efficios.com> <20180129202020.8515-3-mathieu.desnoyers@efficios.com> <8b200dd5-f37b-b208-82fb-2775df7bcd49@csgroup.eu>
Subject: Re: [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory
 barrier in switch_mm()
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Thread-Topic: powerpc: membarrier: Skip memory barrier in switch_mm()
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----- On Jun 18, 2021, at 1:13 PM, Christophe Leroy christophe.leroy@csgroup.eu wrote:
[...]
> 
> I don't understand all that complexity to just replace a simple
> 'smp_mb__after_unlock_lock()'.
> 
> #define smp_mb__after_unlock_lock()	smp_mb()
> #define smp_mb()	barrier()
> # define barrier() __asm__ __volatile__("": : :"memory")
> 
> 
> Am I missing some subtility ?

On powerpc CONFIG_SMP, smp_mb() is actually defined as:

#define smp_mb()        __smp_mb()
#define __smp_mb()      mb()
#define mb()   __asm__ __volatile__ ("sync" : : : "memory")

So the original motivation here was to skip a "sync" instruction whenever
switching between threads which are part of the same process. But based on
recent discussions, I suspect my implementation may be inaccurately doing
so though.

Thanks,

Mathieu


> 
> Thanks
> Christophe

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com