Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

[Vitaly Kuznetsov]

Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:

> On Wed, 12 Jun 2019, Vitaly Kuznetsov wrote:
>> Dmitry Safonov <dima@xxxxxxxxxx> writes:
>> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
>> > index 1608050e9df9..0bdd79ecbff8 100644
>> > --- a/arch/x86/hyperv/hv_init.c
>> > +++ b/arch/x86/hyperv/hv_init.c
>> > @@ -91,7 +91,7 @@ EXPORT_SYMBOL_GPL(hv_max_vp_index);
>> >  static int hv_cpu_init(unsigned int cpu)
>> >  {
>> >  	u64 msr_vp_index;
>> > -	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
>> > +	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[cpu];
>> >  	void **input_arg;
>> >  	struct page *pg;
>> >  
>> > @@ -103,7 +103,7 @@ static int hv_cpu_init(unsigned int cpu)
>> >  
>> >  	hv_get_vp_index(msr_vp_index);
>> >  
>> > -	hv_vp_index[smp_processor_id()] = msr_vp_index;
>> > +	hv_vp_index[cpu] = msr_vp_index;
>> >  
>> >  	if (msr_vp_index > hv_max_vp_index)
>> >  		hv_max_vp_index = msr_vp_index;
>> 
>> The above is unrelated cleanup (as cpu == smp_processor_id() for
>> CPUHP_AP_ONLINE_DYN callbacks), right? As I'm pretty sure these can'd be
>> preempted.
>
> They can be preempted, but they are guaranteed to run on the upcoming CPU,
> i.e. smp_processor_id() is allowed even in preemptible context as the task
> cannot migrate.
>

Ah, right, thanks! The guarantee that they don't migrate should be enough.

-- 
Vitaly