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Date:   Wed, 15 Apr 2020 11:36:17 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Qais Yousef <qais.yousef@arm.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yury Norov <yury.norov@gmail.com>,
        Paul Turner <pjt@google.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Josh Don <joshdon@google.com>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] cpumask: Make cpumask_any() truly random
Message-ID: <20200415093617.GZ20730@hirez.programming.kicks-ass.net>
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 <20200414150556.10920-3-qais.yousef@arm.com>
 <20200414121956.3687d6e9@gandalf.local.home>
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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
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.