[PATCH v2 1/2] cpu-hotplug: export cpu_hotplug_enable/cpu_hotplug_disable

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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@redhat.com>
---
 Documentation/power/suspend-and-cpuhotplug.txt |  6 +++---
 kernel/cpu.c                                   | 13 ++++++++-----
 2 files changed, 11 insertions(+), 8 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..8f35ee6 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -190,17 +190,19 @@ 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;
+	if (cpu_hotplug_disabled)
+		cpu_hotplug_disabled--;
 	cpu_maps_update_done();
 }
-
+EXPORT_SYMBOL_GPL(cpu_hotplug_enable);
 #endif	/* CONFIG_HOTPLUG_CPU */
 
 /* Need to know about CPUs going up/down? */
@@ -600,7 +602,7 @@ int disable_nonboot_cpus(void)
 	if (!error) {
 		BUG_ON(num_online_cpus() > 1);
 		/* Make sure the CPUs won't be enabled by someone else */
-		cpu_hotplug_disabled = 1;
+		cpu_hotplug_disabled++;
 	} else {
 		pr_err("Non-boot CPUs are not disabled\n");
 	}
@@ -622,7 +624,8 @@ void __ref enable_nonboot_cpus(void)
 
 	/* Allow everyone to use the CPU hotplug again */
 	cpu_maps_update_begin();
-	cpu_hotplug_disabled = 0;
+	if (cpu_hotplug_disabled)
+		cpu_hotplug_disabled--;
 	if (cpumask_empty(frozen_cpus))
 		goto out;
 
-- 
1.9.3