linux-kernel - Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

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Message-ID: <1292014165.2746.9.camel@edumazet-laptop>
Date:	Fri, 10 Dec 2010 21:49:25 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Mikael Pettersson <mikpe@...uu.se>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

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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