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

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Date:	Thu, 16 Jan 2014 11:20:31 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Paasch <christoph.paasch@...ouvain.be>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH net v2] tcp: metrics: Avoid duplicate entries with the
 same destination-IP

On Thu, 2014-01-16 at 20:01 +0100, Christoph Paasch wrote:
> 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@...ouvain.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.
> 
>  net/ipv4/tcp_metrics.c | 51 +++++++++++++++++++++++++++++++-------------------
>  1 file changed, 32 insertions(+), 19 deletions(-)

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks !


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