Re: [RFC 3/5] x86: Add a comment clarifying LDT context switching

[Andy Lutomirski]

On Mon, Oct 20, 2014 at 10:41 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Thu, Oct 16, 2014 at 09:21:42AM -0700, Andy Lutomirski wrote:
>> I think it's the same as in the other case in switch_mm. leave_mm does
>> cpumask_clear_cpu(cpu, mm_cpumask(active_mm)), and, once that has
>> happened, modify_ldt won't send an IPI to this CPU. So, if leave_mm
>> runs, and then another CPU calls modify_ldt on the mm that is in lazy
>> mode here, it won't update our LDT register, so the LDT register and
>> prev->context.ldt might not match.
>
> Ok, let me see if I can follow with an example:
>
> We call leave_mm() on, say, cpu 3 and mm_cpumask(active_mm) has cpu 3 and
> 4 set. Then, on cpu 4 we call modify_ldt on that same mm and there in
> alloc_ldt() we have this:
>
>                 if (!cpumask_equal(mm_cpumask(current->mm),
>                                    cpumask_of(smp_processor_id())))
>                         smp_call_function(flush_ldt, current->mm, 1);
>
> and since we've cleared cpu 3 from the cpumask, we don't flush_ldt()
> on it and there you have the difference.
>
> Am I close?

You're exactly correct, or at least you seem to understand it the way I do :)

--Andy
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