linux-kernel - Re: [PATCH 3/3] [UDP6]: Counter increment on BH mode

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Date:	Sat, 15 Dec 2007 19:43:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Wang Chen <wangchen@...fujitsu.com>,
	Gerrit Renker <gerrit@....abdn.ac.uk>, davem@...emloft.net,
	andi@...stfloor.org, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>
Subject: Re: [PATCH 3/3] [UDP6]: Counter increment on BH mode


* Herbert Xu <herbert@...dor.apana.org.au> wrote:

> Ob Tue, Dec 04, 2007 at 12:17:23AM +1100, Herbert Xu wrote:
> > 
> > Never mind, we already have that in local_t and as Alexey correctly
> > points out, USER is still going to be the expensive variant with the
> > preempt_disable (well until BH gets threaded).  So how about this patch?
> 
> I didn't hear any objections so here is the patch again.
> 
> [SNMP]: Fix SNMP counters with PREEMPT
> 
> The SNMP macros use raw_smp_processor_id() in process context which is 
> illegal because the process may be preempted and then migrated to 
> another CPU.

nit: please use 'invalid' instead of 'illegal'.

> This patch makes it use get_cpu/put_cpu to disable preemption.
> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

> -	(per_cpu_ptr(mib[1], raw_smp_processor_id())->mibs[field]++)
> +	do { \
> +		per_cpu_ptr(mib[1], get_cpu())->mibs[field]++; \
> +		put_cpu(); \
> +	} while (0)

> -	(per_cpu_ptr(mib[1], raw_smp_processor_id())->mibs[field] += addend)
> +	do { \
> +		per_cpu_ptr(mib[1], get_cpu())->mibs[field] += addend; \
> +		put_cpu(); \
> +	} while (0)

we could perhaps introduce stat_smp_processor_id(), which signals that 
the CPU id is used for statistical purposes and does not have to be 
exact? In any case, your patch looks good too.

	Ingo
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