linux-kernel - Re: [PATCH 5/5] net: macb: Fix race between HW and driver

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Message-Id: <20140505.165653.1184059074645793091.davem@davemloft.net>
Date:	Mon, 05 May 2014 16:56:53 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	soren.brinkmann@...inx.com
Cc:	michal.simek@...inx.com, nicolas.ferre@...el.com, git@...inx.com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 5/5] net: macb: Fix race between HW and driver

From: Soren Brinkmann <soren.brinkmann@...inx.com>
Date: Sun,  4 May 2014 15:43:02 -0700

> Under "heavy" RX load, the driver cannot handle the descriptors fast
> enough. In detail, when a descriptor is consumed, its used flag is
> cleared and once the RX budget is consumed all descriptors with a
> cleared used flag are prepared to receive more data. Under load though,
> the HW may constantly receive more data and use those descriptors with a
> cleared used flag before they are actually prepared for next usage.
> 
> The head and tail pointers into the RX-ring should always be valid and
> we can omit clearing and checking of the used flag.
> 
> Signed-off-by: Soren Brinkmann <soren.brinkmann@...inx.com>

Isn't the RX_USED bit the only thing that controls what RX entries
the chip will try to use?

I can't see how you can just remove the RX_USED bit handling
altogether.

The problem actually seems to be that in the current code we clear the
RX_USED bit before we actually reallocate the buffer and set it up.

It should be a bug to see the RX_USED bit set in gem_rx_refill(), and
the only reason why it can happen is exactly because you're clearing it
too early in gem_rx().

This change doesn't seem to be correct, I'm not applying this series
sorry.
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