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Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Christoph Lameter <cl@linux.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Venkatesh Pallipadi <venki@google.com>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Mikael Pettersson <mikpe@it.uu.se>, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        John Stultz <johnstul@us.ibm.com>
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Content-Type: text/plain; charset="UTF-8"
Date: Fri, 10 Dec 2010 21:49:25 +0100
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Le vendredi 10 décembre 2010 à 21:39 +0100, Eric Dumazet a écrit :

> This was exactly my suggestion Christoph.
> 
> I am glad you understand it now.
> 
> 

By the way, we need smp_wmb(), not barrier(), even only the "owner cpu"
can write into its 'percpu' seqcount.

There is nothing special about a seqcount being percpu or a 'global'
one. We must have same memory barrier semantics.

	this_cpu_write_seqcount_begin(&myseqcount);
	this_cpu_add(mydata1, add1);
	this_cpu_add(mydata2, add2);
	this_cpu_inc(mydata3);
	this_cpu_write_seqcount_end(&myseqcount);

We protect the data[1,2,3] set with a seqcount, so need smp_wmb() in
both _begin() and _end()


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