Messages in this thread | | | From | Josh Don <> | Date | Mon, 20 Apr 2020 14:36:49 -0700 | Subject | Re: [PATCH 2/4] cpumask: Make cpumask_any() truly random |
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On Mon, Apr 20, 2020 at 8:43 AM Qais Yousef <qais.yousef@arm.com> 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.
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