cpu-hotplug: convert cpu_hotplug_disabled to a counter

As a prerequisite to exporting cpu_hotplug_enable/cpu_hotplug_disable
functions to modules we need to convert cpu_hotplug_disabled to a counter
to properly support disable -> disable -> enable call sequences. E.g.
after Hyper-V vmbus module (which is supposed to be the first user of
exported cpu_hotplug_enable/cpu_hotplug_disable) did cpu_hotplug_disable()
hibernate path calls disable_nonboot_cpus() and if we hit an error in
_cpu_down() enable_nonboot_cpus() will be called on the failure path (thus
making cpu_hotplug_disabled = 0 and leaving cpu hotplug in 'enabled'
state). Same problem is possible if more than 1 module use
cpu_hotplug_disable/cpu_hotplug_enable on their load/unload paths. When
one of these modules is been unloaded it is logical to leave cpu hotplug
in 'disabled' state.

To support the change we need to increse cpu_hotplug_disabled counter
in disable_nonboot_cpus() unconditionally as all users of
disable_nonboot_cpus() are supposed to do enable_nonboot_cpus() in case
an error was returned.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 13 insertions, 8 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5644ec5582b9..0fca2ba96138 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -191,14 +191,14 @@ 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();
 }
 
 void cpu_hotplug_enable(void)
 {
         cpu_maps_update_begin();
-        cpu_hotplug_disabled = 0;
+        WARN_ON(--cpu_hotplug_disabled < 0);
         cpu_maps_update_done();
 }
 
@@ -608,13 +608,18 @@ 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;
 }
@@ -633,7 +638,7 @@ 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;