netdev - Re: [PATCH 2/5] net: mvneta: use per_cpu stats to fix an SMP lock up

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Message-ID: <1389550056.31367.186.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sun, 12 Jan 2014 10:07:36 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willy Tarreau <w@....eu>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>
Subject: Re: [PATCH 2/5] net: mvneta: use per_cpu stats to fix an SMP lock up

On Sun, 2014-01-12 at 10:31 +0100, Willy Tarreau wrote:
> Stats writers are mvneta_rx() and mvneta_tx(). They don't lock anything
> when they update the stats, and as a result, it randomly happens that
> the stats freeze on SMP if two updates happen during stats retrieval.

Your patch is OK, but I dont understand how this freeze can happen.

TX and RX uses a separate syncp, and TX is protected by a lock, RX
is protected by NAPI bit.

Stats retrieval uses the appropriate BH disable before the fetches...


> This is very easily reproducible by starting two HTTP servers and binding
> each of them to a different CPU, then consulting /proc/net/dev in loops
> during transfers, the interface should immediately lock up. This issue
> also randomly happens upon link state changes during transfers, because
> the stats are collected in this situation, but it takes more attempts to
> reproduce it.


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