stop_machine: Make cpu_stop_queue_work() and stop_one_cpu_nowait() return bool

Change cpu_stop_queue_work() to return true if the work was queued and
change stop_one_cpu_nowait() to return the result of cpu_stop_queue_work().
This makes it more useful, for example now you can alloc cpu_stop_work for
stop_one_cpu_nowait() and free it in the callback or if stop_one_cpu_nowait()
fails, currently this is impossible because you can't know if @fn will be
called or not.

Also, this allows to kill cpu_stop_done->executed, see the next changes.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Milos Vyletel <milos@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151117170523.GA13955@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/stop_machine.h

@@ -29,7 +29,7 @@ struct cpu_stop_work {
 
 int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg);
 int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg);
-void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
+bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
 			 struct cpu_stop_work *work_buf);
 int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
 int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
@@ -65,7 +65,7 @@ static void stop_one_cpu_nowait_workfn(struct work_struct *work)
 	preempt_enable();
 }
 
-static inline void stop_one_cpu_nowait(unsigned int cpu,
+static inline bool stop_one_cpu_nowait(unsigned int cpu,
 				       cpu_stop_fn_t fn, void *arg,
 				       struct cpu_stop_work *work_buf)
 {
@@ -74,7 +74,10 @@ static inline void stop_one_cpu_nowait(unsigned int cpu,
 		work_buf->fn = fn;
 		work_buf->arg = arg;
 		schedule_work(&work_buf->work);
+		return true;
 	}
+
+	return false;
 }
 
 static inline int stop_cpus(const struct cpumask *cpumask,

kernel/stop_machine.c

@@ -81,17 +81,21 @@ static void __cpu_stop_queue_work(struct cpu_stopper *stopper,
 }
 
 /* queue @work to @stopper.  if offline, @work is completed immediately */
-static void cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
+static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
 {
 	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 	unsigned long flags;
+	bool enabled;
 
 	spin_lock_irqsave(&stopper->lock, flags);
-	if (stopper->enabled)
+	enabled = stopper->enabled;
+	if (enabled)
 		__cpu_stop_queue_work(stopper, work);
 	else
 		cpu_stop_signal_done(work->done, false);
 	spin_unlock_irqrestore(&stopper->lock, flags);
+
+	return enabled;
 }
 
 /**
@@ -297,12 +301,16 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
  *
  * CONTEXT:
  * Don't care.
+ *
+ * RETURNS:
+ * true if cpu_stop_work was queued successfully and @fn will be called,
+ * false otherwise.
  */
-void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
+bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
 			struct cpu_stop_work *work_buf)
 {
 	*work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
-	cpu_stop_queue_work(cpu, work_buf);
+	return cpu_stop_queue_work(cpu, work_buf);
 }
 
 /* static data for stop_cpus */