linux-kernel - Re: [PATCH 2/3] r8169: Use load_acquire() and store_release() to reduce memory barrier overhead

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Date:	Thu, 13 Nov 2014 22:30:49 +0100
From:	Francois Romieu <romieu@...zoreil.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	linux-arch@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, mikey@...ling.org,
	tony.luck@...el.com, mathieu.desnoyers@...ymtl.ca,
	donald.c.skidmore@...el.com, peterz@...radead.org,
	benh@...nel.crashing.org, heiko.carstens@...ibm.com,
	oleg@...hat.com, will.deacon@....com, davem@...emloft.net,
	michael@...erman.id.au, matthew.vick@...el.com,
	nic_swsd@...ltek.com, geert@...ux-m68k.org,
	jeffrey.t.kirsher@...el.com, fweisbec@...il.com,
	schwidefsky@...ibm.com, linux@....linux.org.uk,
	paulmck@...ux.vnet.ibm.com, torvalds@...ux-foundation.org,
	mingo@...nel.org
Subject: Re: [PATCH 2/3] r8169: Use load_acquire() and store_release() to
 reduce memory barrier overhead

Alexander Duyck <alexander.h.duyck@...hat.com> :
[...]
> In addition the r8169 uses a rmb() however I believe it is placed incorrectly
> as I assume it supposed to be ordering descriptor reads after the check for
> ownership.

Not exactly. It's a barrier against compiler optimization from 2004.
It should not matter.

However I disagree with the change below:

> @@ -7284,11 +7280,11 @@ static int rtl_rx(struct net_device *dev, struct rtl8169_private *tp, u32 budget
>  		struct RxDesc *desc = tp->RxDescArray + entry;
>  		u32 status;
>  
> -		rmb();
> -		status = le32_to_cpu(desc->opts1) & tp->opts1_mask;
> -
> +		status = cpu_to_le32(load_acquire(&desc->opts1));
>  		if (status & DescOwn)
>  			break;
> +
> +		status &= tp->opts1_mask;

-> tp->opts1_mask is not __le32 tainted.

Btw, should I consider the sketch above as a skeleton in my r8169 closet ?

           NIC                      CPU0                      CPU1
| CPU | NIC | CPU | CPU | 

                          | CPU | NIC | CPU | CPU |
                                         ^ tx_dirty

                                [start_xmit...

| CPU | CPU | CPU | CPU |
   (NIC did it's job)
                                                           [rtl_tx...
                          | ... | ... | NIC | NIC |
                                  (ring update)
                              (tx_dirty increases)

                                                     | CPU | CPU | ??? | ??? |
                                                           tx_dirty ?
                                                     reaping about-to-be-sent
                                                     buffers on some platforms ?
                                ...start_xmit]


-- 
Ueimor
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