[PATCH] cpufreq: schedutil: Fix sugov_start versus sugov_update_shared race

[Vikram Mulukutla <markivx@codeaurora.org>]

With a shared policy in place, when one of the CPUs in the policy is
hotplugged out and then brought back online, sugov_stop and
sugov_start are called in order.

sugov_stop removes utilization hooks for each CPU in the policy and
does nothing else in the for_each_cpu loop. sugov_start on the other
hand iterates through the CPUs in the policy and re-initializes the
per-cpu structure _and_ adds the utilization hook. This implies that
the scheduler is allowed to invoke a CPU's utilization update hook
when the rest of the per-cpu structures have yet to be re-inited.

Apart from some strange values in tracepoints this doesn't cause a
problem, but if we do end up accessing a pointer from the per-cpu
sugov_cpu structure somewhere in the sugov_update_shared path,
we will likely see crashes since the memset for another CPU in the
policy is free to race with sugov_update_shared from the CPU that is
ready to go. So let's fix this now to first init all per-cpu
structures, and then add the per-cpu utilization update hooks all at
once.

Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
---
 kernel/sched/cpufreq_schedutil.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 076a2e3..29a3970 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -610,6 +610,11 @@ static int sugov_start(struct cpufreq_policy *policy)
 		sg_cpu->sg_policy = sg_policy;
 		sg_cpu->flags = SCHED_CPUFREQ_RT;
 		sg_cpu->iowait_boost_max = policy->cpuinfo.max_freq;
+	}
+
+	for_each_cpu(cpu, policy->cpus) {
+		struct sugov_cpu *sg_cpu = &per_cpu(sugov_cpu, cpu);
+
 		cpufreq_add_update_util_hook(cpu, &sg_cpu->update_util,
 					     policy_is_shared(policy) ?
 							sugov_update_shared :
-- 
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