Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into separate phases and document them

Thomas Gleixner
2023-05-09T12:07:24Z

On Tue, May 09 2023 at 12:04, Peter Zijlstra wrote:
> On Mon, May 08, 2023 at 09:43:39PM +0200, Thomas Gleixner wrote:
>> + /*
>> + * Sync point with wait_cpu_callin(). The AP doesn't wait here
>> + * but just sets the bit to let the controlling CPU (BSP) know that
>> + * it's got this far.
>> + */
>> smp_callin();
>>
>> - /* otherwise gcc will move up smp_processor_id before the cpu_init */
>> + /* Otherwise gcc will move up smp_processor_id() before cpu_init() */
>> barrier();
>
> Not to the detriment of this patch, but this barrier() and it's comment
> seem weird vs smp_callin(). That function ends with an atomic bitop (it
> has to, at the very least it must not be weaker than store-release) but
> also has an explicit wmb() to order setup vs CPU_STARTING.
>
> (arguably that should be a full fence *AND* get a comment)
>
> There is no way the smp_processor_id() referred to in this comment can
> land before cpu_init() even without the barrier().

Right. Let me clean that up.

Thanks,

tglx