linux-kernel - Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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Date:	Thu, 19 Feb 2009 14:00:24 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Oleg Nesterov <oleg@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())

On Thu, 2009-02-19 at 04:20 -0800, Ingo Molnar wrote:
> Could you please refresh this patch to latest tip:master? The 
> APIC drivers moved to arch/x86/kernel/apic/.

Appended the refreshed patch. Thanks.
---

From: Suresh Siddha <suresh.b.siddha@...el.com>
Subject: x86: move smp_mb() in flush tlb path to x2apic specific paths

uncached MMIO accesses for xapic are inherently serializing and hence
we don't need explicit barriers for xapic IPI paths.

x2apic MSR writes/reads don't have serializing semantics and hence need
a serializing instruction or mfence, to make all the previous memory
stores globally visisble before the x2apic msr write for IPI.

And hence move smp_mb() in x86 flush tlb path to x2apic specific paths.

Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
---

diff --git a/arch/x86/kernel/apic/x2apic_cluster.c b/arch/x86/kernel/apic/x2apic_cluster.c
index 3f7df23..c54fffe 100644
--- a/arch/x86/kernel/apic/x2apic_cluster.c
+++ b/arch/x86/kernel/apic/x2apic_cluster.c
@@ -63,6 +63,13 @@ static void x2apic_send_IPI_mask(const struct cpumask *mask, int vector)
 	unsigned long query_cpu;
 	unsigned long flags;
 
+	/*
+	 * Make previous memory operations globally visible before
+	 * sending the IPI. We need a serializing instruction or mfence
+	 * for this.
+	 */
+	smp_mb();
+
 	local_irq_save(flags);
 	for_each_cpu(query_cpu, mask) {
 		__x2apic_send_IPI_dest(
@@ -79,6 +86,13 @@ static void
 	unsigned long query_cpu;
 	unsigned long flags;
 
+	/*
+	 * Make previous memory operations globally visible before
+	 * sending the IPI. We need a serializing instruction or mfence
+	 * for this.
+	 */
+	smp_mb();
+
 	local_irq_save(flags);
 	for_each_cpu(query_cpu, mask) {
 		if (query_cpu == this_cpu)
@@ -96,6 +110,13 @@ static void x2apic_send_IPI_allbutself(int vector)
 	unsigned long query_cpu;
 	unsigned long flags;
 
+	/*
+	 * Make previous memory operations globally visible before
+	 * sending the IPI. We need a serializing instruction or mfence
+	 * for this.
+	 */
+	smp_mb();
+
 	local_irq_save(flags);
 	for_each_online_cpu(query_cpu) {
 		if (query_cpu == this_cpu)
diff --git a/arch/x86/kernel/apic/x2apic_phys.c b/arch/x86/kernel/apic/x2apic_phys.c
index d2d52eb..84cc2f3 100644
--- a/arch/x86/kernel/apic/x2apic_phys.c
+++ b/arch/x86/kernel/apic/x2apic_phys.c
@@ -58,6 +58,13 @@ static void x2apic_send_IPI_mask(const struct cpumask *mask, int vector)
 	unsigned long query_cpu;
 	unsigned long flags;
 
+	/*
+	 * Make previous memory operations globally visible before
+	 * sending the IPI. We need a serializing instruction or mfence
+	 * for this.
+	 */
+	smp_mb();
+
 	local_irq_save(flags);
 	for_each_cpu(query_cpu, mask) {
 		__x2apic_send_IPI_dest(per_cpu(x86_cpu_to_apicid, query_cpu),
@@ -73,6 +80,13 @@ static void
 	unsigned long query_cpu;
 	unsigned long flags;
 
+	/*
+	 * Make previous memory operations globally visible before
+	 * sending the IPI. We need a serializing instruction or mfence
+	 * for this.
+	 */
+	smp_mb();
+
 	local_irq_save(flags);
 	for_each_cpu(query_cpu, mask) {
 		if (query_cpu != this_cpu)
@@ -89,6 +103,13 @@ static void x2apic_send_IPI_allbutself(int vector)
 	unsigned long query_cpu;
 	unsigned long flags;
 
+	/*
+	 * Make previous memory operations globally visible before
+	 * sending the IPI. We need a serializing instruction or mfence
+	 * for this.
+	 */
+	smp_mb();
+
 	local_irq_save(flags);
 	for_each_online_cpu(query_cpu) {
 		if (query_cpu == this_cpu)
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index a654d59..821e970 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -187,11 +187,6 @@ static void flush_tlb_others_ipi(const struct cpumask *cpumask,
 		       cpumask, cpumask_of(smp_processor_id()));
 
 	/*
-	 * Make the above memory operations globally visible before
-	 * sending the IPI.
-	 */
-	smp_mb();
-	/*
 	 * We have to send the IPI only to
 	 * CPUs affected.
 	 */


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