netdev - RE: [PATCH net-next 3/5] be2net: fix erx->rx_drops_no

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Date:	Mon, 22 Aug 2011 05:15:45 -0700
From:	<Sathya.Perla@...lex.Com>
To:	<eric.dumazet@...il.com>
CC:	<netdev@...r.kernel.org>
Subject: RE: [PATCH net-next 3/5] be2net: fix erx->rx_drops_no_frags wrap
 around

>-----Original Message-----
>From: Eric Dumazet [mailto:eric.dumazet@...il.com]
>Sent: Monday, August 22, 2011 5:18 PM
>
>Le lundi 22 août 2011 à 13:43 +0200, Eric Dumazet a écrit :
>
>> This adds a race for lockless SNMP readers (in be_get_stats64())
>>
>> They can see the 32bit value going backward.
>>
>> You have to be very careful to read the *acc once, and write it once.
>
>The "write once" is the only requirement.
>
>Something like :
>
>u32 newval = hi(*acc) + val;
>
>if (wrapped)
>	newval += 65536;
>ACCESS_ONCE(*acc) = newval;
>

Eric,

I'm wondering why you'd need ACCESS_ONCE() here? Wouldn't using the temp variable (newval) as you suggested,
ensure that the reader doesn't see a half-baked value?