netdev - Re: [PATCH v2] l2tp: use per-cpu variables for u64

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Message-ID: <4FEB6B64.5060708@hp.com>
Date:	Wed, 27 Jun 2012 13:21:56 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Tom Parkin <tparkin@...alix.com>, netdev@...r.kernel.org,
	David.Laight@...LAB.COM, James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH v2] l2tp: use per-cpu variables for u64_stats updates

On 06/27/2012 12:03 PM, Eric Dumazet wrote:
> On Wed, 2012-06-27 at 13:00 +0100, Tom Parkin wrote:
>> This patch fixes a race condition in l2tp when updating tunnel and
>> session statistics.  Previously it was possible for multiple threads
>> to concurrently call u64_stats_update*(), which lead to statistics
>> readers blocking forever.
>>
>> This race was discovered on an AMD64 SMP machine running a 32bit
>> kernel.  Running "ip l2tp" while sending data over an Ethernet
>> pseudowire resulted in an occasional soft lockup in
>> u64_stats_fetch_begin() called from l2tp_nl_session_send().
>>
>> For safe lockless update of l2tp stats, data is now stored in per-cpu
>> variables.  These per-cpu datasets are then summed at read time via.
>> an extra helper function l2tp_stats_copy() which has been added to
>> l2tp_core.c.
>>
>
> Do we really need 64bits stats on 32bit arches for l2tp ?

It is a question of the speed of the communications more than the 
bitness of the processor no?

rick jones

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