Date: Fri, 10 Dec 2010 13:51:23 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
To: Peter Zijlstra <peterz@infradead.org>
cc: Eric Dumazet <eric.dumazet@gmail.com>,
        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>
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
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On Fri, 10 Dec 2010, Peter Zijlstra wrote:

> > > gcc wont be able to do this yet (%fs/%gs selectors)
> >
> > The kernel can do that using the __percpu annotation.
>
> That's not true:
>
> # define __percpu
>
> Its a complete NOP.

The annotation serves for sparse checking. .... If you do not care about
those checks then you can simply pass a percpu pointer in the same form as
a regular pointer.

> > > But we can provide this_cpu_write_seqcount_{begin|end}()
> >
> > No we cannot do hat. this_cpu ops are for per cpu data and not for locking
> > values shared between processors. We have a mechanism for passing per cpu
> > pointers with a corresponding annotation.
>
> -enoparse, its not locking anything, is a per-cpu sequence count.

seqlocks are for synchronization of objects on different processors.

Seems that you do not have that use case in mind. So a seqlock restricted
to a single processor? If so then you wont need any of those smp write
barriers mentioned earlier. A simple compiler barrier() is sufficient.


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