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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>
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        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
In-Reply-To: <20230509100421.GU83892@hirez.programming.kicks-ass.net>
References: <20230508181633.089804905@linutronix.de>
 <20230508185217.671595388@linutronix.de>
 <20230509100421.GU83892@hirez.programming.kicks-ass.net>
Date:   Tue, 09 May 2023 14:07:20 +0200
Message-ID: <87pm791zev.ffs@tglx>
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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