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Date:   Thu, 15 Jun 2023 10:58:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Mario Limonciello <mario.limonciello@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Tony Battersby <tonyb@cybernetics.com>,
        Ashok Raj <ashok.raj@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Arjan van de Veen <arjan@linux.intel.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Ashok Raj <ashok.raj@intel.com>
Subject: Re: [patch V2 3/8] x86/smp: Remove pointless wmb() from
 native_stop_other_cpus()
Message-ID: <20230615085828.GC1683497@hirez.programming.kicks-ass.net>
References: <20230613115353.599087484@linutronix.de>
 <20230613121615.762734722@linutronix.de>
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On Tue, Jun 13, 2023 at 02:17:58PM +0200, Thomas Gleixner wrote:
> The wmb() after the successfull atomic_cmpxchg() is complete voodoo along
> with the comment stating "sync above data before sending IRQ".
> 
> There is no "above" data except for the atomic_t stopping_cpu which has
> just been acquired. The reboot IPI handler does not check any data and
> unconditionally disables the CPU.
> 
> Remove this cargo cult barrier.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Ashok Raj <ashok.raj@intel.com>
> ---
>  arch/x86/kernel/smp.c |    3 ---
>  1 file changed, 3 deletions(-)
> 
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -174,9 +174,6 @@ static void native_stop_other_cpus(int w
>  
>  		atomic_set(&stop_cpus_count, num_online_cpus() - 1);
>  
> -		/* sync above data before sending IRQ */
> -		wmb();
> -
>  		apic_send_IPI_allbutself(REBOOT_VECTOR);

There's a second one a little below. That too should go.

More to the point, the apic_send_*() functions should be the ones that
ensure this if required etc.. See for example weak_wrmsr_fence() for
x2apic.