linux-kernel - [PATCH v5 1/2] cpu-hotplug: export cpu_hotplug_enable/cpu_hotplug

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Message-Id: <1435240249-8893-2-git-send-email-vkuznets@redhat.com>
Date:	Thu, 25 Jun 2015 15:50:48 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	devel@...uxdriverproject.org
Cc:	"K. Y. Srinivasan" <kys@...rosoft.com>,
	linux-kernel@...r.kernel.org, Dexuan Cui <decui@...rosoft.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Radim Krčmář <rkrcmar@...hat.com>,
	gregkh@...uxfoundation.org
Subject: [PATCH v5 1/2] cpu-hotplug: export cpu_hotplug_enable/cpu_hotplug_disable

Loaded Hyper-V module will use these functions to disable CPU hotplug
under certain circumstances. Convert cpu_hotplug_disabled to a counter
(protected by cpu_add_remove_lock) to support e.g. disable -> disable ->
enable call sequences.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
 Documentation/power/suspend-and-cpuhotplug.txt |  6 +++---
 kernel/cpu.c                                   | 24 +++++++++++++++---------
 2 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/Documentation/power/suspend-and-cpuhotplug.txt b/Documentation/power/suspend-and-cpuhotplug.txt
index 2850df3..2fc9095 100644
--- a/Documentation/power/suspend-and-cpuhotplug.txt
+++ b/Documentation/power/suspend-and-cpuhotplug.txt
@@ -72,7 +72,7 @@ More details follow:
                                         |
                                         v
                            Disable regular cpu hotplug
-                        by setting cpu_hotplug_disabled=1
+                        by increasing cpu_hotplug_disabled
                                         |
                                         v
                             Release cpu_add_remove_lock
@@ -89,7 +89,7 @@ Resuming back is likewise, with the counterparts being (in the order of
 execution during resume):
 * enable_nonboot_cpus() which involves:
    |  Acquire cpu_add_remove_lock
-   |  Reset cpu_hotplug_disabled to 0, thereby enabling regular cpu hotplug
+   |  Decrease cpu_hotplug_disabled, thereby enabling regular cpu hotplug
    |  Call _cpu_up() [for all those cpus in the frozen_cpus mask, in a loop]
    |  Release cpu_add_remove_lock
    v
@@ -120,7 +120,7 @@ after the entire cycle is complete (i.e., suspend + resume).
                            Acquire cpu_add_remove_lock
                                         |
                                         v
-                          If cpu_hotplug_disabled is 1
+                          If cpu_hotplug_disabled > 0
                                 return gracefully
                                         |
                                         |
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 94bbe46..cab3c92 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -190,17 +190,18 @@ void cpu_hotplug_done(void)
 void cpu_hotplug_disable(void)
 {
 	cpu_maps_update_begin();
-	cpu_hotplug_disabled = 1;
+	cpu_hotplug_disabled++;
 	cpu_maps_update_done();
 }
+EXPORT_SYMBOL_GPL(cpu_hotplug_disable);
 
 void cpu_hotplug_enable(void)
 {
 	cpu_maps_update_begin();
-	cpu_hotplug_disabled = 0;
+	WARN_ON(--cpu_hotplug_disabled < 0);
 	cpu_maps_update_done();
 }
-
+EXPORT_SYMBOL_GPL(cpu_hotplug_enable);
 #endif	/* CONFIG_HOTPLUG_CPU */
 
 /* Need to know about CPUs going up/down? */
@@ -597,13 +598,18 @@ int disable_nonboot_cpus(void)
 		}
 	}
 
-	if (!error) {
+	if (!error)
 		BUG_ON(num_online_cpus() > 1);
-		/* Make sure the CPUs won't be enabled by someone else */
-		cpu_hotplug_disabled = 1;
-	} else {
+	else
 		pr_err("Non-boot CPUs are not disabled\n");
-	}
+
+	/*
+	 * Make sure the CPUs won't be enabled by someone else. We need to do
+	 * this even in case of failure as all disable_nonboot_cpus() users are
+	 * supposed to do enable_nonboot_cpus() on the failure path.
+	 */
+	cpu_hotplug_disabled++;
+
 	cpu_maps_update_done();
 	return error;
 }
@@ -622,7 +628,7 @@ void __ref enable_nonboot_cpus(void)
 
 	/* Allow everyone to use the CPU hotplug again */
 	cpu_maps_update_begin();
-	cpu_hotplug_disabled = 0;
+	WARN_ON(--cpu_hotplug_disabled < 0);
 	if (cpumask_empty(frozen_cpus))
 		goto out;
 
-- 
2.4.3

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