Re: [PATCH v2 3/3] stop_machine: change cpu_stop_queue_two_works() to rely on stopper->enabled

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 08, 2015 at 07:01:41PM +0200, Oleg Nesterov wrote:
> @@ -261,12 +276,8 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
>  	set_state(&msdata, MULTI_STOP_PREPARE);
>  
>  	/*
> -	 * If we observe both CPUs active we know _cpu_down() cannot yet have
> -	 * queued its stop_machine works and therefore ours will get executed
> -	 * first. Or its not either one of our CPUs that's getting unplugged,
> -	 * in which case we don't care.
> -	 *
> -	 * This relies on the stopper workqueues to be FIFO.
> +	 * We do not want to migrate to inactive CPU. FIXME: move this
> +	 * into migrate_swap_stop() callback.
>  	 */
>  	if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
>  		preempt_enable();


I stuck that on top.. I'll have a closer look at the 7 patches later
when I might be more coherent (mad head-ache atm.)

---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1335,12 +1335,16 @@ static int migrate_swap_stop(void *data)
 	struct rq *src_rq, *dst_rq;
 	int ret = -EAGAIN;
 
+	if (!cpu_active(arg->src_cpu) || !cpu_active(arg->dst_cpu))
+		return -EAGAIN;
+
 	src_rq = cpu_rq(arg->src_cpu);
 	dst_rq = cpu_rq(arg->dst_cpu);
 
 	double_raw_lock(&arg->src_task->pi_lock,
 			&arg->dst_task->pi_lock);
 	double_rq_lock(src_rq, dst_rq);
+
 	if (task_cpu(arg->dst_task) != arg->dst_cpu)
 		goto unlock;
 
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -275,15 +275,6 @@ int stop_two_cpus(unsigned int cpu1, uns
 	cpu_stop_init_done(&done, 2);
 	set_state(&msdata, MULTI_STOP_PREPARE);
 
-	/*
-	 * We do not want to migrate to inactive CPU. FIXME: move this
-	 * into migrate_swap_stop() callback.
-	 */
-	if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
-		preempt_enable();
-		return -ENOENT;
-	}
-
 	if (cpu1 > cpu2)
 		swap(cpu1, cpu2);
 	if (cpu_stop_queue_two_works(cpu1, &work1, cpu2, &work2)) {