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

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Date:	Mon, 22 Aug 2011 14:44:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sathya.Perla@...lex.Com
Cc:	netdev@...r.kernel.org
Subject: RE: [PATCH net-next 3/5] be2net: fix erx->rx_drops_no_frags wrap
 around

Le lundi 22 août 2011 à 05:15 -0700, Sathya.Perla@...lex.Com a écrit :
> >-----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?
> 

If you write :

u32 newval = hi(*acc) + val;
if (wrapped)
	newval += 65536;
*acc = newval;


C compiler (gcc) is free to eliminate the temp variable and to generate
same code than :

*acc = hi(*acc) + val;
if (wrapped)
	*acc += 65536;

ACCESS_ONCE() here is the clean way (and self explanatory/documented) to
keep compiler to write to *acc twice.



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