linux-kernel - [patch v3 3/7] x86/smp: Remove pointless wmb()s from native_stop_other

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Message-ID: <20230615193330.378358382@linutronix.de>
Date:   Thu, 15 Jun 2023 22:33:54 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     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>
Subject: [patch v3 3/7] x86/smp: Remove pointless wmb()s from native_stop_other_cpus()

The wmb()s before sending the IPIs are not synchronizing anything.

If at all then the apic IPI functions have to provide or act as appropriate
barriers.

Remove these cargo cult barriers which have no explanation of what they are
synchronizing.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
V3: Remove second instance and reword changelog - PeterZ
---
 arch/x86/kernel/smp.c |    6 ------
 1 file changed, 6 deletions(-)

--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -184,9 +184,6 @@ static void native_stop_other_cpus(int w
 	cpumask_clear_cpu(cpu, &cpus_stop_mask);
 
 	if (!cpumask_empty(&cpus_stop_mask)) {
-		/* sync above data before sending IRQ */
-		wmb();
-
 		apic_send_IPI_allbutself(REBOOT_VECTOR);
 
 		/*
@@ -210,9 +207,6 @@ static void native_stop_other_cpus(int w
 		if (!smp_no_nmi_ipi && !register_stop_handler()) {
 			u32 dm;
 
-			/* Sync above data before sending IRQ */
-			wmb();
-
 			pr_emerg("Shutting down cpus with NMI\n");
 
 			dm = apic->dest_mode_logical ? APIC_DEST_LOGICAL : APIC_DEST_PHYSICAL;