Re: [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state racy load (v2)

[Oleg Nesterov]

On 09/08, Mathieu Desnoyers wrote:
>
> +static void sync_runqueues_membarrier_state(struct mm_struct *mm)
> +{
> +	int membarrier_state = atomic_read(&mm->membarrier_state);
> +	bool fallback = false;
> +	cpumask_var_t tmpmask;
> +	int cpu;
> +
> +	if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1) {
> +		WRITE_ONCE(this_rq()->membarrier_state, membarrier_state);

This doesn't look safe, this caller can migrate to another CPU after
it calculates the per-cpu ptr.

I think you need do disable preemption or simply use this_cpu_write().

Oleg.