netdev - Re: [PATCH net-next-2.6] net: Introduce u64_stats

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Message-ID: <1276603972.2541.99.camel@edumazet-laptop>
Date:	Tue, 15 Jun 2010 14:12:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	bhutchings@...arflare.com
Subject: Re: [PATCH net-next-2.6] net: Introduce u64_stats_sync
 infrastructure

Le mardi 15 juin 2010 à 21:04 +1000, Nick Piggin a écrit :

> So if you have a regular seqlock, the sequence:
> 
> write_seqcount_begin
> stat1++
> stat2--
> write_seqcount_end
> 
> do read_seqcount_begin
>   sum = stat1+stat2;
> while (read_seqcount_retry)
> BUG_ON(sum != 0);
> 
> This code is OK. But if it is using your stat sync, then it is buggy.
> This is obvious to you of course, but someone who doesn't consider
> the implementation might get caught out.
> 
> I guess all my other points are properties of seqcount code itself,
> but they are not documented really well with the seqlock API
> unfortunately.
> 

Yep, not counting fact that stat_sync is a noop on 64 bits anyway, so
external synchronization is pretty much needed (and probably already
present on all path were I need this stuff. If not, it must be fixed)



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