Re: [PATCH 2/4] cpumask: Make cpumask_any() truly random

[Qais Yousef]

On 04/15/20 11:36, Peter Zijlstra wrote:
> On Tue, Apr 14, 2020 at 12:19:56PM -0400, Steven Rostedt wrote:
> 
> > > +/**
> > > + * cpumask_any - pick a "random" cpu from *srcp
> > > + * @srcp: the input cpumask
> > > + *
> > > + * Returns >= nr_cpu_ids if no cpus set.
> > > + */
> > > +int cpumask_any(const struct cpumask *srcp)
> > > +{
> > > +	int next, prev;
> > > +
> > > +	/* NOTE: our first selection will skip 0. */
> > > +	prev = __this_cpu_read(distribute_cpu_mask_prev);
> > > +
> > > +	next = cpumask_next(prev, srcp);
> > > +	if (next >= nr_cpu_ids)
> > > +		next = cpumask_first(srcp);
> > > +
> > > +	if (next < nr_cpu_ids)
> > > +		__this_cpu_write(distribute_cpu_mask_prev, next);
> > 
> > Do we care if this gets preempted and migrated to a new CPU where we read
> > "prev" from one distribute_cpu_mask_prev on one CPU and write it to another
> > CPU?
> 
> I don't think we do; that just adds to the randomness ;-), but you do

Yep we don't care and it should enhance the randomness.

> raise a good point in that __this_cpu_*() ops assume preemption is
> already disabled, which is true of the one exiting
> cpumask_any_and_distribute() caller, but is no longer true after patch
> 1, and this patch repeats the mistake.
> 
> So either we need to disable preemption across the function or
> transition to this_cpu_*() ops.

Sorry wasn't aware about the preemption check in __this_cpu_write().

Transitioning to this_cpu_write() makes sense. Unless Josh comes back it'll
break something he noticed.

Thanks

--
Qais Yousef