padata: Avoid nested calls to cpus_read_lock() in pcrypt_init_padata()

pcrypt_init_padata()
   cpus_read_lock()
   padata_alloc_possible()
     padata_alloc()
       cpus_read_lock()

The nested call to cpus_read_lock() works with the current implementation,
but prevents the conversion to a percpu rwsem.

The other caller of padata_alloc_possible() is pcrypt_init_padata() which
calls from a cpus_read_lock() protected region as well.

Remove the cpus_read_lock() call in padata_alloc() and document the
calling convention.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-crypto@vger.kernel.org
Link: http://lkml.kernel.org/r/20170524081547.571278910@linutronix.de

kernel/padata.c

@@ -940,6 +940,8 @@ static struct kobj_type padata_attr_type = {
  * @wq: workqueue to use for the allocated padata instance
  * @pcpumask: cpumask that will be used for padata parallelization
  * @cbcpumask: cpumask that will be used for padata serialization
+ *
+ * Must be called from a cpus_read_lock() protected region
  */
 static struct padata_instance *padata_alloc(struct workqueue_struct *wq,
 					    const struct cpumask *pcpumask,
@@ -952,7 +954,6 @@ static struct padata_instance *padata_alloc(struct workqueue_struct *wq,
 	if (!pinst)
 		goto err;
 
-	get_online_cpus();
 	if (!alloc_cpumask_var(&pinst->cpumask.pcpu, GFP_KERNEL))
 		goto err_free_inst;
 	if (!alloc_cpumask_var(&pinst->cpumask.cbcpu, GFP_KERNEL)) {
@@ -976,14 +977,12 @@ static struct padata_instance *padata_alloc(struct workqueue_struct *wq,
 
 	pinst->flags = 0;
 
-	put_online_cpus();
-
 	BLOCKING_INIT_NOTIFIER_HEAD(&pinst->cpumask_change_notifier);
 	kobject_init(&pinst->kobj, &padata_attr_type);
 	mutex_init(&pinst->lock);
 
 #ifdef CONFIG_HOTPLUG_CPU
-	cpuhp_state_add_instance_nocalls(hp_online, &pinst->node);
+	cpuhp_state_add_instance_nocalls_cpuslocked(hp_online, &pinst->node);
 #endif
 	return pinst;
 
@@ -992,7 +991,6 @@ static struct padata_instance *padata_alloc(struct workqueue_struct *wq,
 	free_cpumask_var(pinst->cpumask.cbcpu);
 err_free_inst:
 	kfree(pinst);
-	put_online_cpus();
 err:
 	return NULL;
 }
@@ -1003,9 +1001,12 @@ static struct padata_instance *padata_alloc(struct workqueue_struct *wq,
  *                         parallel workers.
  *
  * @wq: workqueue to use for the allocated padata instance
+ *
+ * Must be called from a cpus_read_lock() protected region
  */
 struct padata_instance *padata_alloc_possible(struct workqueue_struct *wq)
 {
+	lockdep_assert_cpus_held();
 	return padata_alloc(wq, cpu_possible_mask, cpu_possible_mask);
 }
 EXPORT_SYMBOL(padata_alloc_possible);