workqueues: make get_online_cpus() useable for work->func()

workqueue_cpu_callback(CPU_DEAD) flushes cwq->thread under
cpu_maps_update_begin().  This means that the multithreaded workqueues
can't use get_online_cpus() due to the possible deadlock, very bad and
very old problem.

Introduce the new state, CPU_POST_DEAD, which is called after
cpu_hotplug_done() but before cpu_maps_update_done().

Change workqueue_cpu_callback() to use CPU_POST_DEAD instead of CPU_DEAD.
This means that create/destroy functions can't rely on get_online_cpus()
any longer and should take cpu_add_remove_lock instead.

[akpm@linux-foundation.org: fix CONFIG_SMP=n]
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Gautham R Shenoy <ego@in.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Max Krasnyansky <maxk@qualcomm.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Paul Menage <menage@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vegard Nossum <vegard.nossum@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 14 insertions, 9 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2cc409ce0a8f..10ba5f1004a5 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -285,6 +285,11 @@ out_allowed:
         set_cpus_allowed_ptr(current, &old_allowed);
 out_release:
         cpu_hotplug_done();
+        if (!err) {
+                if (raw_notifier_call_chain(&cpu_chain, CPU_POST_DEAD | mod,
+                                            hcpu) == NOTIFY_BAD)
+                        BUG();
+        }
         return err;
 }
 
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5fbffd302eb5..828e58230cbc 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -828,7 +828,7 @@ struct workqueue_struct *__create_workqueue_key(const char *name,
                 err = create_workqueue_thread(cwq, singlethread_cpu);
                 start_workqueue_thread(cwq, -1);
         } else {
-                get_online_cpus();
+                cpu_maps_update_begin();
                 spin_lock(&workqueue_lock);
                 list_add(&wq->list, &workqueues);
                 spin_unlock(&workqueue_lock);
@@ -840,7 +840,7 @@ struct workqueue_struct *__create_workqueue_key(const char *name,
                         err = create_workqueue_thread(cwq, cpu);
                         start_workqueue_thread(cwq, cpu);
                 }
-                put_online_cpus();
+                cpu_maps_update_done();
         }
 
         if (err) {
@@ -854,8 +854,8 @@ EXPORT_SYMBOL_GPL(__create_workqueue_key);
 static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq)
 {
         /*
-         * Our caller is either destroy_workqueue() or CPU_DEAD,
-         * get_online_cpus() protects cwq->thread.
+         * Our caller is either destroy_workqueue() or CPU_POST_DEAD,
+         * cpu_add_remove_lock protects cwq->thread.
          */
         if (cwq->thread == NULL)
                 return;
@@ -865,7 +865,7 @@ static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq)
 
         flush_cpu_workqueue(cwq);
         /*
-         * If the caller is CPU_DEAD and cwq->worklist was not empty,
+         * If the caller is CPU_POST_DEAD and cwq->worklist was not empty,
          * a concurrent flush_workqueue() can insert a barrier after us.
          * However, in that case run_workqueue() won't return and check
          * kthread_should_stop() until it flushes all work_struct's.
@@ -889,14 +889,14 @@ void destroy_workqueue(struct workqueue_struct *wq)
         const cpumask_t *cpu_map = wq_cpu_map(wq);
         int cpu;
 
-        get_online_cpus();
+        cpu_maps_update_begin();
         spin_lock(&workqueue_lock);
         list_del(&wq->list);
         spin_unlock(&workqueue_lock);
 
         for_each_cpu_mask_nr(cpu, *cpu_map)
                 cleanup_workqueue_thread(per_cpu_ptr(wq->cpu_wq, cpu));
-        put_online_cpus();
+        cpu_maps_update_done();
 
         free_percpu(wq->cpu_wq);
         kfree(wq);
@@ -935,7 +935,7 @@ static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
 
                 case CPU_UP_CANCELED:
                         start_workqueue_thread(cwq, -1);
-                case CPU_DEAD:
+                case CPU_POST_DEAD:
                         cleanup_workqueue_thread(cwq);
                         break;
                 }
@@ -943,7 +943,7 @@ static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
 
         switch (action) {
         case CPU_UP_CANCELED:
-        case CPU_DEAD:
+        case CPU_POST_DEAD:
                 cpu_clear(cpu, cpu_populated_map);
         }