netdev - Re: [PATCH] bnx2x: fix occasional statistics off-by-4GB error

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Date:	Sun, 17 Mar 2013 22:53:28 +0200
From:	"Eilon Greenstein" <eilong@...adcom.com>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
cc:	"David Miller" <davem@...emloft.net>, dmitry@...adcom.com,
	zenczykowski@...il.com, maze@...gle.com, netdev@...r.kernel.org,
	yuvalmin@...adcom.com
Subject: Re: [PATCH] bnx2x: fix occasional statistics off-by-4GB error

On Sun, 2013-03-17 at 13:25 -0700, Eric Dumazet wrote:
> On Sun, 2013-03-17 at 21:37 +0200, Eilon Greenstein wrote:
> > On Sun, 2013-03-17 at 14:24 -0400, David Miller wrote:
> > > From: "Dmitry Kravkov" <dmitry@...adcom.com>
> > > Date: Sun, 17 Mar 2013 13:10:37 +0000
> > > 
> > > > Probably this commit resolved the issue:
> > > > 
> > > > commit bef05406ac0ea6f468e1e25e9934f3011ea9259b
> > > > Author: Dmitry Kravkov <dmitry@...adcom.com>
> > > > Date:   Tue Sep 11 04:34:08 2012 +0000
> > > > 
> > > >     bnx2x: Avoid sending multiple statistics queries
> > > > 
> > > > Can you try it pls?
> > > 
> > > These are completely seperate bugs.
> > > 
> > > The macro in question does not handle rollover of the lower 32-bits of
> > > the statistic properly at all, and therefore Maciej's patch should be
> > > applied and queud up for -stable.
> > > 
> > > Please give it your ACK unless you can find a bug in his change.
> > 
> > Both the high value and the low value are read from the chip - so this
> > patch will increment the higher 32 bits twice (well, more than once).
> > Not taking the HW/FW high counter at all is also not acceptable since
> > the reading frequency is not high enough without those.
> > 
> > So the original patch is nacked, but we are trying to figure out what is
> > causing the statistics to misbehave and it might be related to sending
> > statistics query twice.
> 
> This looks like the typical problem of updating a 64bit value in non
> atomic way.
> 
> Its guaranteed to happen on 32bit hosts.
> We had to introduce include/linux/u64_stats_sync.h to help to solve this
> without adding extra cost on 64bit arches.
> 
> In bnx2x case, we perform 32bit operations even on 64bit host, so we
> probably need to add a seqcount_t, so that a stat consumer can detect it
> read a stale value.

This is not such a trivial issue - the HW/FW is guaranteeing the atomic
read and this is why we can always use 32b variables.


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