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Date:   Tue, 9 May 2023 12:19:02 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        David Woodhouse <dwmw2@infradead.org>,
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Subject: Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into
 separate phases and document them
Message-ID: <20230509101902.GV83892@hirez.programming.kicks-ass.net>
References: <20230508181633.089804905@linutronix.de>
 <20230508185217.671595388@linutronix.de>
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Again, not really this patch, but since I had to look at this code ....

On Mon, May 08, 2023 at 09:43:39PM +0200, Thomas Gleixner wrote:
> @@ -1048,60 +1066,89 @@ static int do_boot_cpu(int apicid, int c

	/*
	 * AP might wait on cpu_callout_mask in cpu_init() with
	 * cpu_initialized_mask set if previous attempt to online
	 * it timed-out. Clear cpu_initialized_mask so that after
	 * INIT/SIPI it could start with a clean state.
	 */
	cpumask_clear_cpu(cpu, cpu_initialized_mask);
	smp_mb();

^^^ that barrier is weird too, cpumask_clear_cpu() is an atomic op and
implies much the same (this is x86 after all). If you want to be super
explicit about it write:

	smp_mb__after_atomic();

(which is a no-op) but then it still very much requires a comment as to
what exactly it orders against what.


	/*
	 * Wake up a CPU in difference cases:
	 * - Use a method from the APIC driver if one defined, with wakeup
	 *   straight to 64-bit mode preferred over wakeup to RM.
	 * Otherwise,
>  	 * - Use an INIT boot APIC message
>  	 */
>  	if (apic->wakeup_secondary_cpu_64)
> +		return apic->wakeup_secondary_cpu_64(apicid, start_ip);
>  	else if (apic->wakeup_secondary_cpu)
> +		return apic->wakeup_secondary_cpu(apicid, start_ip);
>  
> +	return wakeup_secondary_cpu_via_init(apicid, start_ip);
> +}