Re: [PATCH v2 1/8] RISC-V: Do not wait indefinitely in __cpu_up

Christoph Hellwig
2019-01-18T07:20:53Z

On Thu, Jan 17, 2019 at 06:35:39PM -0800, Atish Patra wrote:
> On 1/15/19 5:51 AM, Christoph Hellwig wrote:
> > > void *__cpu_up_stack_pointer[NR_CPUS];
> > > void *__cpu_up_task_pointer[NR_CPUS];
> > > +static DECLARE_COMPLETION(cpu_running);
> > > void __init smp_prepare_boot_cpu(void)
> > > {
> > > @@ -81,6 +82,7 @@ void __init setup_smp(void)
> > > int __cpu_up(unsigned int cpu, struct task_struct *tidle)
> > > {
> > > + int ret = 0;
> > > int hartid = cpuid_to_hartid_map(cpu);
> > > tidle->thread_info.cpu = cpu;
> > > @@ -96,10 +98,15 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
> > > task_stack_page(tidle) + THREAD_SIZE);
> > > WRITE_ONCE(__cpu_up_task_pointer[hartid], tidle);
> > > - while (!cpu_online(cpu))
> > > - cpu_relax();
> > > + wait_for_completion_timeout(&cpu_running,
> > > + msecs_to_jiffies(1000));
> >
> > Having a global completion here worries me. I bet we have some higher
> > level serialization, but can we comment or even better lockdep assert on
> > that?
> >
>
> Yes. It is serialized from smp.c in smp_init(). It brings one cpu online
> at a time for preset_cpu mask.
>
> Do we still need a lockdep assert ?

I guess the real lock is through cpu_hotplug_lock. And yes, a comment
or even better lockdep assert would be good.