netdev - Re: [PATCH net-next] packet: fix warnings in rollover lock contention

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Message-ID: <1431620686.27831.63.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 14 May 2015 09:24:46 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] packet: fix warnings in rollover lock
 contention

On Thu, 2015-05-14 at 11:53 -0400, Willem de Bruijn wrote:

> I principally want to avoid the lock contention on sk_receive_queue.lock,
> which is held for a lot longer while probing frames. But yes, I'd prefer to
> avoid the cacheline contention as well.
> 
> The alternative is to keep the race and just replace the xchg with a
> straight assignment.

Please describe the race. It seems quite innocent at first look.

Clearly putting xchg() gives a false sense of security in this context.

Atomic ops should be reserved for cases we cannot avoid them,
not to give false hopes ;)



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