Re: [PATCHv6 21/30] x86/acpi, x86/boot: Add multiprocessor wake-up support

Dave Hansen
2022-03-16T23:47:48Z

On 3/15/22 19:08, Kirill A. Shutemov wrote:
> + * The value of apic_id and wakeup_vector has to be set before updating
> + * the wakeup command. To let compiler preserve order of writes, use
> + * smp_store_release.
> + */

Yeah, but that's not what's written:

> + smp_store_release(&acpi_mp_wake_mailbox->apic_id, apicid);
> + smp_store_release(&acpi_mp_wake_mailbox->wakeup_vector, start_ip);
> + smp_store_release(&acpi_mp_wake_mailbox->command,
> + ACPI_MP_WAKE_COMMAND_WAKEUP);

That says that the write to ->apic_id has to happen before the write to
->wakeup_vector which has to happen before the write to ->command. What
you have here *works*, but it doesn't match the comment.

If the problem were the compiler alone, I think three WRITE_ONCE()'s
would also suffice. (Hint: WRITE_ONCE() is insufficient).

I _think_ this will do:

acpi_mp_wake_mailbox->apic_id = apicid;
acpi_mp_wake_mailbox->wakeup_vector = start_ip;
smp_wmb();
WRITE_ONCE(acpi_mp_wake_mailbox->command, ACPI_MP_WAKE_CO...);

But it's the end of the day and I'm sending this out under duress, so
please double-check my logic.

Also, in all practicality, the WRITE_ONCE() isn't going to do much.
->command is 2 bytes and even the stupidest compiler isn't going to
break that up. The compiler also fundamentally understands the ordering
between this ->command write and the below:

READ_ONCE(acpi_mp_wake_mailbox->command).

The READ_ONCE() will also ensure that *it* goes out to memory. *But*,
the WRITE_ONCE() does make it very clear what is supposed to happen.