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

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Message-ID: <9f1aea6e-29ad-33e0-8ec8-c112956491b6@intel.com>
Date:   Wed, 16 Mar 2022 16:47:25 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCHv6 21/30] x86/acpi, x86/boot: Add multiprocessor wake-up
 support

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.