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Message-ID: <20230615085828.GC1683497@hirez.programming.kicks-ass.net>
Date: Thu, 15 Jun 2023 10:58:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
Mario Limonciello <mario.limonciello@....com>,
Tom Lendacky <thomas.lendacky@....com>,
Tony Battersby <tonyb@...ernetics.com>,
Ashok Raj <ashok.raj@...ux.intel.com>,
Tony Luck <tony.luck@...el.com>,
Arjan van de Veen <arjan@...ux.intel.com>,
Eric Biederman <ebiederm@...ssion.com>,
Ashok Raj <ashok.raj@...el.com>
Subject: Re: [patch V2 3/8] x86/smp: Remove pointless wmb() from
native_stop_other_cpus()
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@...utronix.de>
> Reviewed-by: Ashok Raj <ashok.raj@...el.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.
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