Re: [PATCH] sched/core: add rcu_read_lock/unlock() protection

[Peter Zijlstra]

On Tue, Jan 26, 2021 at 04:46:51PM +0800, qiang.zhang@xxxxxxxxxxxxx wrote:
> From: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> 
> Due to for_each_process_thread belongs to RCU read operation,
> need to add rcu_read_lock/unlock() protection.
> 
> Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8c5481077c9c..c3f0103fdf53 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7738,6 +7738,7 @@ static void dump_rq_tasks(struct rq *rq, const char *loglvl)
>  	lockdep_assert_held(&rq->lock);
>  
>  	printk("%sCPU%d enqueued tasks (%u total):\n", loglvl, cpu, rq->nr_running);
> +	rcu_read_lock();
>  	for_each_process_thread(g, p) {
>  		if (task_cpu(p) != cpu)
>  			continue;
> @@ -7747,6 +7748,7 @@ static void dump_rq_tasks(struct rq *rq, const char *loglvl)
>  
>  		printk("%s\tpid: %d, name: %s\n", loglvl, p->pid, p->comm);
>  	}
> +	rcu_read_unlock();

We're in stop machine, with IRQs disabled, please explain how this can
make any difference?