linux-kernel - Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

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Date:   Sun, 03 Mar 2019 20:05:23 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Burton <paul.burton@...s.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Tony Luck <tony.luck@...el.com>,
        Will Deacon <will.deacon@....com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of
 mmiowb() tracking

Linus Torvalds's on March 3, 2019 2:29 pm:
> On Sat, Mar 2, 2019, 19:34 Nicholas Piggin <npiggin@...il.com> wrote:
> 
>>
>> It doesn't have to be done all at once with this series, obviously this
>> is a big improvement on its own. But why perpetuate the nomenclature
>> and concept for new code added now?
>>
> 
> What nomenclature?
> 
> Nobody will be using mmiowb(). That's the whole point of the patch series.
> 
> It's now an entirely internal name, and nobody cares.

Why even bother with it at all, "internal" or not?  Just get rid of 
mmiowb, the concept is obsolete.

> And none of this has anything to do with wmb(), since it's about IO being
> ordered across cpu's by spin locks, not by barriers.
> 
> So I'm not seeing what you're arguing about.

Pretend ia64 doesn't exist for a minute. Now the regular mb/wmb barriers 
orders IO across CPUs with respect to their cacheable accesses.  
Regardless of whether that cacheable access is a spin lock, a bit lock, 
an atomic, a mutex... This is how it was before mmiowb came along.

Nothing wrong with this series to make spinlocks order mmio, but why 
call it mmiowb? Another patch could rename ia64's mmiowb and then the
name can be removed from the tree completely.

Thanks,
Nick