Re: [PATCH net v2] tcp: metrics: Avoid duplicate entries with the same destination-IP

[David Miller <davem@davemloft.net>]

From: Christoph Paasch <christoph.paasch@uclouvain.be>
Date: Thu, 16 Jan 2014 20:01:21 +0100

> Because the tcp-metrics is an RCU-list, it may be that two
> soft-interrupts are inside __tcp_get_metrics() for the same
> destination-IP at the same time. If this destination-IP is not yet part of
> the tcp-metrics, both soft-interrupts will end up in tcpm_new and create
> a new entry for this IP.
> So, we will have two tcp-metrics with the same destination-IP in the list.
> 
> This patch checks twice __tcp_get_metrics(). First without holding the
> lock, then while holding the lock. The second one is there to confirm
> that the entry has not been added by another soft-irq while waiting for
> the spin-lock.
> 
> Fixes: 51c5d0c4b169b (tcp: Maintain dynamic metrics in local cache.)
> Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
> ---
> 
> v2: As requested by Eric D.: Check the cache twice. Once without holding the lock,
>     and then again while holding the lock. That way we avoid taking the lock
>     needlessly.

Looks great, applied and queued up for -stable, thanks.