1 files changed, 33 insertions, 0 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 3619e939182e..d61ba88f34e5 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -21,6 +21,24 @@ EXPORT_SYMBOL_GPL(cpucontrol);
 
 static struct notifier_block *cpu_chain;
 
+/*
+ * Used to check by callers if they need to acquire the cpucontrol
+ * or not to protect a cpu from being removed. Its sometimes required to
+ * call these functions both for normal operations, and in response to
+ * a cpu being added/removed. If the context of the call is in the same
+ * thread context as a CPU hotplug thread, we dont need to take the lock
+ * since its already protected
+ * check drivers/cpufreq/cpufreq.c for its usage - Ashok Raj
+ */
+
+int current_in_cpu_hotplug(void)
+{
+        return (current->flags & PF_HOTPLUG_CPU);
+}
+
+EXPORT_SYMBOL_GPL(current_in_cpu_hotplug);
+
+
 /* Need to know about CPUs going up/down? */
 int register_cpu_notifier(struct notifier_block *nb)
 {
@@ -94,6 +112,13 @@ int cpu_down(unsigned int cpu)
                 goto out;
         }
 
+        /*
+         * Leave a trace in current->flags indicating we are already in
+         * process of performing CPU hotplug. Callers can check if cpucontrol
+         * is already acquired by current thread, and if so not cause
+         * a dead lock by not acquiring the lock
+         */
+        current->flags |= PF_HOTPLUG_CPU;
         err = notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
                                                 (void *)(long)cpu);
         if (err == NOTIFY_BAD) {
@@ -146,6 +171,7 @@ out_thread:
 out_allowed:
         set_cpus_allowed(current, old_allowed);
 out:
+        current->flags &= ~PF_HOTPLUG_CPU;
         unlock_cpu_hotplug();
         return err;
 }
@@ -163,6 +189,12 @@ int __devinit cpu_up(unsigned int cpu)
                 ret = -EINVAL;
                 goto out;
         }
+
+        /*
+         * Leave a trace in current->flags indicating we are already in
+         * process of performing CPU hotplug.
+         */
+        current->flags |= PF_HOTPLUG_CPU;
         ret = notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu);
         if (ret == NOTIFY_BAD) {
                 printk("%s: attempt to bring up CPU %u failed\n",
@@ -185,6 +217,7 @@ out_notify:
         if (ret != 0)
                 notifier_call_chain(&cpu_chain, CPU_UP_CANCELED, hcpu);
 out:
+        current->flags &= ~PF_HOTPLUG_CPU;
         up(&cpucontrol);
         return ret;
 }