netdev - Re: [PATCH -next] net: tcp: add mib counters to track zero window transitions

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Message-ID: <1392839209.18849.20.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 19 Feb 2014 11:46:49 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Florian Westphal <fw@...len.de>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH -next] net: tcp: add mib counters to track zero window
 transitions

On Wed, 2014-02-19 at 20:30 +0100, Hannes Frederic Sowa wrote:

> NET_STATS only use 32 bit counter and thus need not be protected with a
> seqlock on 32 bit platforms. As such, it does not matter, but e.g. the IP
> counter are prone to deadlocks if used with wrong postfix because of 64 bit
> counter thus protected by seqlock.
> 
> A pitty that the _STATS_BH postfixes have the opposite meaning of the bottom
> _bh postfixes.
> 
> Basically always safe is _STATS and if we are sure we can omit the bh disable
> call because we only call the function from bh, we can use _STATS_BH calls.

Using __this_cpu_inc() is not safe in preemptable contexts.

Sure, x86 doesn't care, but other arches might.

Fortunately LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES can be incremented
only for a retransmit, and they should always happen from BH.

Better avoid the confusion for this ultra rare case, I'll send a patch.


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