linux-kernel - Re: [PATCH v2 17/21] drivers: Remove explicit invocations of mmiowb()

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Date:   Tue, 09 Apr 2019 19:00:52 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>
Cc:     Akira Yokosawa <akiyks@...il.com>,
        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>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        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>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH v2 17/21] drivers: Remove explicit invocations of mmiowb()

Linus Torvalds's on April 6, 2019 1:50 am:
> On Fri, Apr 5, 2019 at 4:01 AM Will Deacon <will.deacon@....com> wrote:
>>
>> mmiowb() is now implied by spin_unlock() on architectures that require
>> it, so there is no reason to call it from driver code. This patch was
>> generated using coccinelle:
>>
>>         @mmiowb@
>>         @@
>>         - mmiowb();
> 
> So I love the patch series, and think we should just do it, but I do
> wonder if some of the drivers involved end up relying on memory
> ordering things (store_release -> load_aquire) and IO ordering rather
> than using locking...

Hopefully the convention that smp_ prefix does not work for MMIO
ordering helps there. Drivers relying on that would be broken today
on powerpc, at least.

> Wouldn't such use now be broken on ia64 SN platforms? Do we care?

Hopefully not too much, what changed since last thread? :)

> So it might be worth noting that a lot of the mmiowb()s here weren't
> paired with spin_unlock?

I repeat myself, but the correct change is for ia64 to #define wmb to
mmiowb, then nothing is silently broken, nothing has to be noted, and 
nobody has to care. The ia64/sn2 platform will run a little slower 
that's all.

But deliberately breaking sn2 I guess is implicitly acknowledging the 
same end result that I wanted, so fine.

I think it might be an idea to remove all the mmiowb() that obviously
come before spin_unlock in one big patch, but then submit the rest 
individually to driver maintainers. I could do that rather than ask
more work from Will, if he and you agree.

Thanks,
Nick