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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Apr 16, 2014 at 10:08:03AM -0500, Christoph Lameter wrote:
> 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.

Cool, this is a good short-term fix.

> > 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.

Understood!

							Thanx, Paul