linux-kernel - Re: LKMM: Read dependencies of writes ordered by dma

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Date:   Wed, 18 Aug 2021 16:17:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Marco Elver <elver@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: LKMM: Read dependencies of writes ordered by dma_wmb()?

On Wed, Aug 18, 2021 at 12:39:36PM +0100, Will Deacon wrote:
> Hi Paul.

Hello, Will,

> On Tue, Aug 17, 2021 at 06:53:08AM -0700, Paul E. McKenney wrote:
> > On Tue, Aug 17, 2021 at 01:28:16PM +0100, Will Deacon wrote:

[ . . . ]

> > > Ignore the bits about mmiowb() as we got rid of that.
> > 
> > Should the leftovers in current mainline be replaced by wmb()?  Or are
> > patches to that effect on their way in somewhere?
> 
> I already got rid of the non-arch usage of mmiowb(), but I wasn't bravei
> enough to change the arch code as it may well be that they're relying on
> some specific instruction semantics.
> 
> Despite my earlier comment, mmiowb() still exists, but only as a part of
> ARCH_HAS_MMIOWB where it is used to add additional spinlock ordering so
> that the rest of the kernel doesn't need to use mmiowb() at all.
> 
> So I suppose for these:
> 
> > arch/mips/kernel/gpio_txx9.c:	mmiowb();
> > arch/mips/kernel/gpio_txx9.c:	mmiowb();
> > arch/mips/kernel/gpio_txx9.c:	mmiowb();
> > arch/mips/kernel/irq_txx9.c:	mmiowb();
> > arch/mips/loongson2ef/common/bonito-irq.c:	mmiowb();
> > arch/mips/loongson2ef/common/bonito-irq.c:	mmiowb();
> > arch/mips/loongson2ef/common/mem.c:		mmiowb();
> > arch/mips/loongson2ef/common/pm.c:	mmiowb();
> > arch/mips/loongson2ef/lemote-2f/reset.c:	mmiowb();
> > arch/mips/loongson2ef/lemote-2f/reset.c:	mmiowb();
> > arch/mips/loongson2ef/lemote-2f/reset.c:	mmiowb();
> > arch/mips/loongson2ef/lemote-2f/reset.c:	mmiowb();
> > arch/mips/loongson2ef/lemote-2f/reset.c:	mmiowb();
> > arch/mips/pci/ops-bonito64.c:	mmiowb();
> > arch/mips/pci/ops-loongson2.c:	mmiowb();
> > arch/mips/txx9/generic/irq_tx4939.c:	mmiowb();
> > arch/mips/txx9/generic/setup.c:	mmiowb();
> > arch/mips/txx9/rbtx4927/irq.c:	mmiowb();
> > arch/mips/txx9/rbtx4938/irq.c:	mmiowb();
> > arch/mips/txx9/rbtx4938/irq.c:	mmiowb();
> > arch/mips/txx9/rbtx4938/setup.c:	mmiowb();
> > arch/mips/txx9/rbtx4939/irq.c:	mmiowb();
> 
> we could replace mmiowb() with iobarrier_w().

Not having MIPS hardware at my disposal, I will leave these to those
who do.  I would suggest adding iobarrier_*() to memory-barriers.txt,
but they appear to be specific to MIPS and PowerPC.

							Thanx, Paul