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Date: Tue, 11 Nov 2014 13:12:32 -0800
From: Alexander Duyck <alexander.h.duyck@...hat.com>
To: Will Deacon <will.deacon@....com>,
"alexander.duyck@...il.com" <alexander.duyck@...il.com>
CC: "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Michael Neuling <mikey@...ling.org>,
Tony Luck <tony.luck@...el.com>,
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
Peter Zijlstra <peterz@...radead.org>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Oleg Nesterov <oleg@...hat.com>,
Michael Ellerman <michael@...erman.id.au>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
Russell King <linux@....linux.org.uk>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] arch: Introduce read_acquire()
On 11/11/2014 11:47 AM, Will Deacon wrote:
> Hello,
>
> On Tue, Nov 11, 2014 at 06:57:05PM +0000, alexander.duyck@...il.com wrote:
>> From: Alexander Duyck <alexander.h.duyck@...hat.com>
>>
>> In the case of device drivers it is common to utilize receive descriptors
>> in which a single field is used to determine if the descriptor is currently
>> in the possession of the device or the CPU. In order to prevent any other
>> fields from being read a rmb() is used resulting in something like code
>> snippet from ixgbe_main.c:
>>
>> if (!ixgbe_test_staterr(rx_desc, IXGBE_RXD_STAT_DD))
>> break;
>>
>> /*
>> * This memory barrier is needed to keep us from reading
>> * any other fields out of the rx_desc until we know the
>> * RXD_STAT_DD bit is set
>> */
>> rmb();
>>
>> On reviewing the documentation and code for smp_load_acquire() it occured
>> to me that implementing something similar for CPU <-> device interraction
>> would be worth while. This commit provides just the load/read side of this
>> in the form of read_acquire(). This new primative orders the specified
>> read against any subsequent reads. As a result we can reduce the above
>> code snippet down to:
>>
>> /* This memory barrier is needed to keep us from reading
>> * any other fields out of the rx_desc until we know the
>> * RXD_STAT_DD bit is set
>> */
>> if (!(read_acquire(&rx_desc->wb.upper.status_error) &
> Minor nit on naming, but load_acquire would match what we do with barriers,
> where you simply drop the smp_ prefix if you want the thing to work on UP
> systems too.
The problem is this is slightly different, load_acquire in my mind would
use a mb() call, I only use a rmb(). That is why I chose read_acquire
as the name.
>> cpu_to_le32(IXGBE_RXD_STAT_DD)))
>> break;
> I'm not familiar with the driver in question, but how are the descriptors
> mapped? Is the read barrier here purely limiting re-ordering of normal
> memory accesses by the CPU? If so, isn't there also scope for store_release
> when updating, e.g. next_to_watch in the same driver?
So the driver in question is using descriptor rings allocated via
dma_alloc_coherent. The device is notified that new descriptors are
present via a memory mapped I/O register, then the device will read the
descriptor via a DMA operation and then write it back with another DMA
operation and the process of doing so it will set the IXGBE_RXD_STAT_DD bit.
The problem with the store_release logic is that it would need to key
off of a write to memory mapped I/O. The idea had crossed my mind, but
I wasn't confident I had a good enough understanding of things to try
and deal with memory ordering for cacheable and uncachable memory in the
same call. I would have to do some more research to see if something
like that is even possible as I suspect some of the architectures may
not support something like that.
> We also need to understand how this plays out with
> smp_mb__after_unlock_lock, which is currently *only* implemented by PowerPC.
> If we end up having a similar mess to mmiowb, where PowerPC both implements
> the barrier *and* plays tricks in its spin_unlock code, then everybody
> loses because we'd end up with release doing the right thing anyway.
PowerPC is not much of a risk in this patch. The implementation I did
just fell back to a rmb().
The architectures I need to sort out are arm, x86, sparc, ia64, and s390
as they are the only ones that tried to make use of the smp_load_acquire
logic.
> Peter and I spoke with Paul at LPC about strengthening
> smp_load_acquire/smp_store_release so that release->acquire ordering is
> maintained, which would allow us to drop smp_mb__after_unlock_lock
> altogether. That's stronger than acquire/release in C11, but I think it's
> an awful lot easier to use, particularly if device drivers are going to
> start using these primitives.
>
> Thoughts?
>
> Will
I generally want just enough of a barrier in place to keep things
working properly without costing much in terms of CPU time. If you can
come up with a generic load_acquire/store_release that could take the
place of this function I am fine with that as long as it would function
at the same level of performance.
Thanks,
Alex
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