Re: How do I increment a per-CPU variable without warning?

[Christoph Lameter <cl@linux.com>]

On Tue, 15 Apr 2014, Paul E. McKenney wrote:

> Hello, Christoph,
>
> I have a patch that currently uses __this_cpu_inc_return() to increment a
> per-CPU variable, but without preemption disabled.  Of course, given that
> preemption is enabled, it might well end up picking up one CPU's counter,
> adding one to it, then storing the result into some other CPU's counter.
> But this is OK, the test can be probabilistic.  And when I run this
> against v3.14 and earlier, it works fine.

We introduced raw_cpu_inc_return to squish these warnings.

> This is arguably better than the original __this_cpu_read() because it
> avoids overflow, but I thought I should check to see if there was some
> better way to do this.

If this is supposed to be totally race safe then you must disable
preemption.