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Date:	Wed, 29 Oct 2014 12:27:48 -0700
From:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] use smp_load_acquire()/smp_store_release()

On Wed, 2014-10-29 at 09:16 -0700, Alexander Duyck wrote:
> On 10/29/2014 07:49 AM, Eric Dumazet wrote:
> > Hi Alexander
> >
> > The memory barriers added in commit
> > b37c0fbe3f6dfba1f8ad2aed47fb40578a254635
> > ("net: Add memory barriers to prevent possible race in byte queue
> > limits")
> >
> > have heavy cost.
> >
> > It seems we could use smp_load_acquire() and smp_store_release()
> > instead ?
> >
> > I'll post a patch later today. I would be interested if someone was able
> > to test it, as your commit apparently was tested and known to fix a
> > reproducible race.
> >
> > Thanks !

Eric- just CC me on the patch you post and I will see what I can do
about getting validation eyes on it.

> 
> Unfortunately Stephen left Intel before I did, so we will need to find 
> someone else in the validation team to test this if possible. I have 
> added Jeff to the CC so that he can give the appropriate validation 
> people a heads up that this patch might be coming.
> 
> As I recall what was seen was random Tx hangs on systems with the 
> original BQL code when interfaces were stressed.  It has been a while so 
> I don't recall the exact set-up for all of it.  Also some less 
> used/tested architectures such as PowerPC can be more susceptible to 
> synchronization issues such as these as the memory model is more weakly 
> ordered.
> 
> I'm wondering where you are seeing the barrier show up?  In 
> netdev_tx_send_queue you should only hit the barrier if you actually are 
> triggering the XOFF condition, and in netdev_tx_completed_queue the 
> barrier should be coalesced in amongst a number of frames reducing the cost.
> 
> My concern with this would be that we are actually syncronizing multiple 
> things, the __QUEUE_STATE_STACK_XOFF flag, dql->adj_limit, and 
> dql->num_queued, and we might be trading off reducing the cost on x86 to 
> result in it being increased on other architectures as we may have to 
> actually add additional synchronization as I suspect we would need to 
> use acquire/release on both adj_limit and num_queued.
> 
> Thanks,
> 
> Alex
> 



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