Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for this_cpu_read/write()

Christoph Lameter
2011-09-20T16:09:01Z

On Tue, 20 Sep 2011, Steven Rostedt wrote:

> On Tue, 2011-09-20 at 09:57 -0500, Christoph Lameter wrote:
>
> > this_cpu_xx functions are made for those locations that have
> > preemption enabled. If you can use those function (classic case is a
> > per cpu counter increment in the network subsystem) then you can avoid
> > preempt disable/enable or get_cpu/put_cpu.
>
> If the variables are used for a very short time, then the latencies
> introduced by a simple:
>
> var = get_cpu_var(my_var);
> if (var)
> do_something_quick();
> put_cpu_var(my_var);
>
> Otherwise if that do_something_quick(); migrates, it may be doing
> something it shouldn't be doing!

That is obviously a use case in which this_cpu_xx ops could not be used
since we must stay on the same cpu. I think there is still a lot of
confusion on your part. The example here shows it.