linux-kernel - Re: [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory barrier in switch

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Date:   Sat, 19 Jun 2021 11:35:34 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        maged michael <maged.michael@...il.com>,
        Dave Watson <davejwatson@...com>,
        Will Deacon <will.deacon@....com>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        David Sehr <sehr@...gle.com>,
        Paul Mackerras <paulus@...ba.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-arch <linux-arch@...r.kernel.org>, x86 <x86@...nel.org>,
        Andrew Hunter <ahh@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Paul <paulmck@...ux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Avi Kivity <avi@...lladb.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        linux-api <linux-api@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory
 barrier in switch_mm()



Le 18/06/2021 à 19:26, Mathieu Desnoyers a écrit :
> ----- On Jun 18, 2021, at 1:13 PM, Christophe Leroy christophe.leroy@...roup.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.
> 

I see.

Then, if you think a 'sync' is a concern, shouldn't we try and remove the forest of 'sync' in the 
I/O accessors ?

I can't really understand why we need all those 'sync' and 'isync' and 'twi' around the accesses 
whereas I/O memory is usually mapped as 'Guarded' so memory access ordering is already garantied.

I'm sure we'll save a lot with that.

Christophe