linux-kernel - Re: [PATCH for 4.16 04/10] membarrier: provide SHARED

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Message-ID: <alpine.DEB.2.20.1801162003070.2366@nanos>
Date:   Tue, 16 Jan 2018 20:04:18 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrea Parri <parri.andrea@...il.com>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Greg Hackmann <ghackmann@...gle.com>,
        Will Deacon <will.deacon@....com>,
        David Sehr <sehr@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        x86 <x86@...nel.org>
Subject: Re: [PATCH for 4.16 04/10] membarrier: provide SHARED_EXPEDITED
 command (v2)

On Tue, 16 Jan 2018, Mathieu Desnoyers wrote:
> ----- On Jan 16, 2018, at 1:20 PM, Thomas Gleixner tglx@...utronix.de wrote:
> > And I really have to ask whether this should be named _GLOBAL_ instead of
> > _SHARED_.
> > 
> > Hmm?
> 
> I agree with you that this behavior fits better a "global" definition
> than a "shared" one, especially given that it does not target a specific
> shared memory mapping. The main issue I have is due to the pre-existing
> MEMBARRIER_CMD_SHARED introduced in Linux 4.3. That one should also have
> been called "MEMBARRIER_CMD_GLOBAL" based on the current line of thoughts.
> 
> Do you envision a way to transition forward to a new "MEMBARRIER_CMD_GLOBAL" for
> the currently existing MEMBARRIER_CMD_SHARED ?
> 
> Perhaps with a duplicated enum entry ?
> 
> enum membarrier_cmd {
>         MEMBARRIER_CMD_QUERY                                    = 0,
>         MEMBARRIER_CMD_SHARED                                   = (1 << 0), /* use MEMBARRIER_CMD_GLOBAL instead */
>         MEMBARRIER_CMD_GLOBAL                                   = (1 << 0),

That should work. Though I doubt that you ever can get rid of CMD_SHARED,
but at least the code is clearer that way.

Thanks,

	tglx