This patch implements process_freezer based cpu-hotplug
core. 
The sailent features are:
o No more (un)lock_cpu_hotplug.
o No more CPU_LOCK_ACQUIRE and CPU_LOCK_RELEASE. Hence no per-subsystem
  hotcpu mutexes.
o Calls freeze_process/thaw_processes at the beginning/end of
  the hotplug operation.
o Splits CPU_DEAD into two events namely
  - CPU_DEAD: which will be handled while the processes are still
              frozen.

  - CPU_DEAD_KILL_THREADS: To be handled after we thaw_processes.

 This split is required because stopping of the per-cpu threads
 using kthread_stop cannot be done while the system is in the frozen
 state. Hence we need subsystems which have created per-cpu threads
 have to stop them once thaw_processes have been called.

o Handles CPU_DEAD and CPU_DEAD_KILL_THREADS for subsystems which
  create per-cpu threads.

Points to ponder: 
o Can calling CPU_DOWN_PREPARE/CPU_UP_PREPARE in the 
frozen context create any dirty dependencies in the future?

o Can the SYSTEM_RUNNING hack in _cpu_up be avoided by some cleaner means.

Signed-off-by : Srivatsa Vaddagiri <[EMAIL PROTECTED]>
Signed-off-by : Gautham R Shenoy   <[EMAIL PROTECTED]>
--
 include/linux/notifier.h |    3 --
 kernel/cpu.c             |   68 +++++++++++++++++++----------------------------
 kernel/sched.c           |    7 +++-
 kernel/softirq.c         |   10 ++++--
 kernel/softlockup.c      |    3 +-
 kernel/workqueue.c       |   16 ++++++++++-
 6 files changed, 58 insertions(+), 49 deletions(-)

Index: hotplug/kernel/cpu.c
===================================================================
--- hotplug.orig/kernel/cpu.c
+++ hotplug/kernel/cpu.c
@@ -14,6 +14,7 @@
 #include <linux/kthread.h>
 #include <linux/stop_machine.h>
 #include <linux/mutex.h>
+#include <linux/freezer.h>
 
 /* This protects CPUs going up and down... */
 static DEFINE_MUTEX(cpu_add_remove_lock);
@@ -28,38 +29,15 @@ static int cpu_hotplug_disabled;
 
 #ifdef CONFIG_HOTPLUG_CPU
 
-/* Crappy recursive lock-takers in cpufreq! Complain loudly about idiots */
-static struct task_struct *recursive;
-static int recursive_depth;
-
 void lock_cpu_hotplug(void)
 {
-       struct task_struct *tsk = current;
-
-       if (tsk == recursive) {
-               static int warnings = 10;
-               if (warnings) {
-                       printk(KERN_ERR "Lukewarm IQ detected in hotplug 
locking\n");
-                       WARN_ON(1);
-                       warnings--;
-               }
-               recursive_depth++;
-               return;
-       }
-       mutex_lock(&cpu_bitmask_lock);
-       recursive = tsk;
+       return;
 }
 EXPORT_SYMBOL_GPL(lock_cpu_hotplug);
 
 void unlock_cpu_hotplug(void)
 {
-       WARN_ON(recursive != current);
-       if (recursive_depth) {
-               recursive_depth--;
-               return;
-       }
-       recursive = NULL;
-       mutex_unlock(&cpu_bitmask_lock);
+       return;
 }
 EXPORT_SYMBOL_GPL(unlock_cpu_hotplug);
 
@@ -133,7 +111,11 @@ static int _cpu_down(unsigned int cpu)
        if (!cpu_online(cpu))
                return -EINVAL;
 
-       raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
+       if (freeze_processes()) {
+               thaw_processes();
+               return -EBUSY;
+       }
+
        err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
                                        hcpu, -1, &nr_calls);
        if (err == NOTIFY_BAD) {
@@ -141,8 +123,8 @@ static int _cpu_down(unsigned int cpu)
                                          nr_calls, NULL);
                printk("%s: attempt to take down CPU %u failed\n",
                                __FUNCTION__, cpu);
-               err = -EINVAL;
-               goto out_release;
+               thaw_processes();
+               return -EINVAL;
        }
 
        /* Ensure that we are not runnable on dying cpu */
@@ -151,9 +133,7 @@ static int _cpu_down(unsigned int cpu)
        cpu_clear(cpu, tmp);
        set_cpus_allowed(current, tmp);
 
-       mutex_lock(&cpu_bitmask_lock);
        p = __stop_machine_run(take_cpu_down, NULL, cpu);
-       mutex_unlock(&cpu_bitmask_lock);
 
        if (IS_ERR(p) || cpu_online(cpu)) {
                /* CPU didn't die: tell everyone.  Can't complain. */
@@ -161,9 +141,12 @@ static int _cpu_down(unsigned int cpu)
                                            hcpu) == NOTIFY_BAD)
                        BUG();
 
+               set_cpus_allowed(current, old_allowed);
+               thaw_processes();
+
                if (IS_ERR(p)) {
                        err = PTR_ERR(p);
-                       goto out_allowed;
+                       return err;
                }
                goto out_thread;
        }
@@ -185,13 +168,12 @@ static int _cpu_down(unsigned int cpu)
 
        check_for_tasks(cpu);
 
+       thaw_processes();
+
+       if (raw_notifier_call_chain(&cpu_chain, CPU_DEAD_KILL_THREADS, hcpu) == 
NOTIFY_BAD)
+               BUG();
 out_thread:
        err = kthread_stop(p);
-out_allowed:
-       set_cpus_allowed(current, old_allowed);
-out_release:
-       raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE,
-                                               (void *)(long)cpu);
        return err;
 }
 
@@ -219,7 +201,12 @@ static int __cpuinit _cpu_up(unsigned in
        if (cpu_online(cpu) || !cpu_present(cpu))
                return -EINVAL;
 
-       raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
+       if (system_state == SYSTEM_RUNNING && freeze_processes()) {
+               if (system_state == SYSTEM_RUNNING)
+                       thaw_processes();
+               return -EBUSY;
+       }
+
        ret = __raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu,
                                                        -1, &nr_calls);
        if (ret == NOTIFY_BAD) {
@@ -229,10 +216,9 @@ static int __cpuinit _cpu_up(unsigned in
                goto out_notify;
        }
 
+
        /* Arch-specific enabling code. */
-       mutex_lock(&cpu_bitmask_lock);
        ret = __cpu_up(cpu);
-       mutex_unlock(&cpu_bitmask_lock);
        if (ret != 0)
                goto out_notify;
        BUG_ON(!cpu_online(cpu));
@@ -241,10 +227,12 @@ static int __cpuinit _cpu_up(unsigned in
        raw_notifier_call_chain(&cpu_chain, CPU_ONLINE, hcpu);
 
 out_notify:
+       if (system_state == SYSTEM_RUNNING)
+               thaw_processes();
+
        if (ret != 0)
                __raw_notifier_call_chain(&cpu_chain,
                                CPU_UP_CANCELED, hcpu, nr_calls, NULL);
-       raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu);
 
        return ret;
 }
Index: hotplug/kernel/softirq.c
===================================================================
--- hotplug.orig/kernel/softirq.c
+++ hotplug/kernel/softirq.c
@@ -18,6 +18,7 @@
 #include <linux/rcupdate.h>
 #include <linux/smp.h>
 #include <linux/tick.h>
+#include <linux/freezer.h>
 
 #include <asm/irq.h>
 /*
@@ -489,7 +490,6 @@ void __init softirq_init(void)
 static int ksoftirqd(void * __bind_cpu)
 {
        set_user_nice(current, 19);
-       current->flags |= PF_NOFREEZE;
 
        set_current_state(TASK_INTERRUPTIBLE);
 
@@ -515,6 +515,9 @@ static int ksoftirqd(void * __bind_cpu)
                        preempt_disable();
                }
                preempt_enable();
+               try_to_freeze();
+               if (cpu_is_offline((long)__bind_cpu))
+                       goto wait_to_die;
                set_current_state(TASK_INTERRUPTIBLE);
        }
        __set_current_state(TASK_RUNNING);
@@ -612,11 +615,12 @@ static int __cpuinit cpu_callback(struct
                kthread_bind(per_cpu(ksoftirqd, hotcpu),
                             any_online_cpu(cpu_online_map));
        case CPU_DEAD:
+               takeover_tasklets(hotcpu);
+               break;
+       case CPU_DEAD_KILL_THREADS:
                p = per_cpu(ksoftirqd, hotcpu);
                per_cpu(ksoftirqd, hotcpu) = NULL;
                kthread_stop(p);
-               takeover_tasklets(hotcpu);
-               break;
 #endif /* CONFIG_HOTPLUG_CPU */
        }
        return NOTIFY_OK;
Index: hotplug/kernel/sched.c
===================================================================
--- hotplug.orig/kernel/sched.c
+++ hotplug/kernel/sched.c
@@ -5519,8 +5519,6 @@ migration_call(struct notifier_block *nf
        case CPU_DEAD:
                migrate_live_tasks(cpu);
                rq = cpu_rq(cpu);
-               kthread_stop(rq->migration_thread);
-               rq->migration_thread = NULL;
                /* Idle task back to normal (off runqueue, low prio) */
                rq = task_rq_lock(rq->idle, &flags);
                deactivate_task(rq->idle, rq);
@@ -5545,6 +5543,11 @@ migration_call(struct notifier_block *nf
                }
                spin_unlock_irq(&rq->lock);
                break;
+       case CPU_DEAD_KILL_THREADS:
+               rq = cpu_rq(cpu);
+               kthread_stop(rq->migration_thread);
+               rq->migration_thread = NULL;
+               break;
 #endif
        case CPU_LOCK_RELEASE:
                mutex_unlock(&sched_hotcpu_mutex);
Index: hotplug/kernel/softlockup.c
===================================================================
--- hotplug.orig/kernel/softlockup.c
+++ hotplug/kernel/softlockup.c
@@ -13,6 +13,7 @@
 #include <linux/kthread.h>
 #include <linux/notifier.h>
 #include <linux/module.h>
+#include <linux/freezer.h>
 
 static DEFINE_SPINLOCK(print_lock);
 
@@ -132,7 +133,7 @@ cpu_callback(struct notifier_block *nfb,
                /* Unbind so it can run.  Fall thru. */
                kthread_bind(per_cpu(watchdog_task, hotcpu),
                             any_online_cpu(cpu_online_map));
-       case CPU_DEAD:
+       case CPU_DEAD_KILL_THREADS:
                p = per_cpu(watchdog_task, hotcpu);
                per_cpu(watchdog_task, hotcpu) = NULL;
                kthread_stop(p);
Index: hotplug/kernel/workqueue.c
===================================================================
--- hotplug.orig/kernel/workqueue.c
+++ hotplug/kernel/workqueue.c
@@ -368,6 +368,7 @@ static int worker_thread(void *__cwq)
        DEFINE_WAIT(wait);
        struct k_sigaction sa;
        sigset_t blocked;
+       int bind_cpu = smp_processor_id();
 
        if (!cwq->wq->freezeable)
                current->flags |= PF_NOFREEZE;
@@ -392,8 +393,11 @@ static int worker_thread(void *__cwq)
        do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);
 
        for (;;) {
-               if (cwq->wq->freezeable)
+               if (cwq->wq->freezeable) {
                        try_to_freeze();
+                       if (cpu_is_offline(bind_cpu))
+                               goto wait_to_die;
+               }
 
                prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE);
                if (!cwq->should_stop && list_empty(&cwq->worklist))
@@ -407,6 +411,16 @@ static int worker_thread(void *__cwq)
        }
 
        return 0;
+
+wait_to_die:
+       /* Wait for kthread_stop */
+       set_current_state(TASK_INTERRUPTIBLE);
+       while (!kthread_should_stop()) {
+               schedule();
+               set_current_state(TASK_INTERRUPTIBLE);
+       }
+       __set_current_state(TASK_RUNNING);
+       return 0;
 }
 
 struct wq_barrier {
Index: hotplug/include/linux/notifier.h
===================================================================
--- hotplug.orig/include/linux/notifier.h
+++ hotplug/include/linux/notifier.h
@@ -194,8 +194,7 @@ extern int __srcu_notifier_call_chain(st
 #define CPU_DOWN_PREPARE       0x0005 /* CPU (unsigned)v going down */
 #define CPU_DOWN_FAILED                0x0006 /* CPU (unsigned)v NOT going 
down */
 #define CPU_DEAD               0x0007 /* CPU (unsigned)v dead */
-#define CPU_LOCK_ACQUIRE       0x0008 /* Acquire all hotcpu locks */
-#define CPU_LOCK_RELEASE       0x0009 /* Release all hotcpu locks */
+#define CPU_DEAD_KILL_THREADS  0x0008 /* Kill per-cpu threads after dead */
 
 #endif /* __KERNEL__ */
 #endif /* _LINUX_NOTIFIER_H */
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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