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

[Josh Don]

On Mon, Apr 20, 2020 at 8:43 AM Qais Yousef <qais.yousef@xxxxxxx> wrote:
>
> On 04/15/20 11:36, Peter Zijlstra wrote:
> > On Tue, Apr 14, 2020 at 12:19:56PM -0400, Steven Rostedt wrote:
> > > 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.

Yep, this_cpu_* makes sense to me.  Preemption is ok, since prev must
always be a valid cpu id, thus we just get a little more _random_ from
this pseudorandom implementation.