workqueue: split kick_pool() into kick_pool_pick()

Factor the worker selection out of kick_pool() into kick_pool_pick(),
which picks and claims the worker under pool->lock but, instead of waking
it, returns the worker's task via an out-param so the caller can issue
the wakeup after dropping pool->lock. BH kicks and wake_cpu setup still
happen under the lock.

kick_pool() becomes a thin wrapper that wakes the returned task, so all
existing callers keep waking under pool->lock.

Pure refactor, no functional change.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -1258,19 +1258,27 @@ static void kick_bh_pool(struct worker_pool *pool)
 }
 
 /**
- * kick_pool - wake up an idle worker if necessary
+ * kick_pool_pick - select an idle worker to kick, deferring the wakeup
  * @pool: pool to kick
+ * @wakep: out-param, set to the task to wake after pool->lock is dropped
  *
- * @pool may have pending work items. Wake up worker if necessary. Returns
- * whether a worker was woken up.
+ * Like kick_pool() but, for a regular (non-BH) pool, returns the picked
+ * worker's task via @wakep instead of waking it, so the caller can issue the
+ * wakeup after dropping pool->lock (the wakeup takes rq->lock). Worker
+ * selection, wake_cpu setup and the BH kick still happen under the lock.
+ * Returns whether a worker was selected or kicked.
+ *
+ * Must be called with @pool->lock held.
  */
-static bool kick_pool(struct worker_pool *pool)
+static bool kick_pool_pick(struct worker_pool *pool, struct task_struct **wakep)
 {
 	struct worker *worker = first_idle_worker(pool);
 	struct task_struct *p;
 
 	lockdep_assert_held(&pool->lock);
 
+	*wakep = NULL;
+
 	if (!need_more_worker(pool) || !worker)
 		return false;
 
@@ -1310,10 +1318,27 @@ static bool kick_pool(struct worker_pool *pool)
 		}
 	}
 #endif
-	wake_up_process(p);
+	*wakep = p;
 	return true;
 }
 
+/**
+ * kick_pool - wake up an idle worker if necessary
+ * @pool: pool to kick
+ *
+ * @pool may have pending work items. Wake up worker if necessary. Returns
+ * whether a worker was woken up.
+ */
+static bool kick_pool(struct worker_pool *pool)
+{
+	struct task_struct *p;
+	bool kicked = kick_pool_pick(pool, &p);
+
+	if (p)
+		wake_up_process(p);
+	return kicked;
+}
+
 #ifdef CONFIG_WQ_CPU_INTENSIVE_REPORT
 
 /*