SUNRPC: Deduplicate thread wake-up code

Refactor: Extract the loop that finds an idle service thread from
svc_xprt_enqueue() and svc_wake_up(). Both functions do just about
the same thing.

Note that svc_wake_up() currently does not hold the RCU read lock
while waking the target thread. It indeed should hold the lock, just
as svc_xprt_enqueue() does, to ensure the rqstp does not vanish
during the wake-up. This patch adds the RCU lock for svc_wake_up().

Note that shrinking the pool thread count is rare, and calls to
svc_wake_up() are also quite infrequent. In practice, this race is
very unlikely to be hit, so we are not marking the lock fix for
stable backport at this time.
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc.h

@@ -419,6 +419,7 @@ int		   svc_register(const struct svc_serv *, struct net *, const int,
 
 void		   svc_wake_up(struct svc_serv *);
 void		   svc_reserve(struct svc_rqst *rqstp, int space);
+bool		   svc_pool_wake_idle_thread(struct svc_pool *pool);
 struct svc_pool   *svc_pool_for_cpu(struct svc_serv *serv);
 char *		   svc_print_addr(struct svc_rqst *, char *, size_t);
 const char *	   svc_proc_name(const struct svc_rqst *rqstp);

net/sunrpc/svc.c

@@ -687,6 +687,40 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 	return rqstp;
 }
 
+/**
+ * svc_pool_wake_idle_thread - Awaken an idle thread in @pool
+ * @pool: service thread pool
+ *
+ * Can be called from soft IRQ or process context. Finding an idle
+ * service thread and marking it BUSY is atomic with respect to
+ * other calls to svc_pool_wake_idle_thread().
+ *
+ * Return value:
+ *   %true: An idle thread was awoken
+ *   %false: No idle thread was found
+ */
+bool svc_pool_wake_idle_thread(struct svc_pool *pool)
+{
+	struct svc_rqst	*rqstp;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(rqstp, &pool->sp_all_threads, rq_all) {
+		if (test_and_set_bit(RQ_BUSY, &rqstp->rq_flags))
+			continue;
+
+		WRITE_ONCE(rqstp->rq_qtime, ktime_get());
+		wake_up_process(rqstp->rq_task);
+		rcu_read_unlock();
+		percpu_counter_inc(&pool->sp_threads_woken);
+		trace_svc_wake_up(rqstp->rq_task->pid);
+		return true;
+	}
+	rcu_read_unlock();
+
+	set_bit(SP_CONGESTED, &pool->sp_flags);
+	return false;
+}
+
 /*
  * Choose a pool in which to create a new thread, for svc_set_num_threads
  */

net/sunrpc/svc_xprt.c

@@ -457,7 +457,6 @@ static bool svc_xprt_ready(struct svc_xprt *xprt)
 void svc_xprt_enqueue(struct svc_xprt *xprt)
 {
 	struct svc_pool *pool;
-	struct svc_rqst	*rqstp = NULL;
 
 	if (!svc_xprt_ready(xprt))
 		return;
@@ -477,20 +476,7 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
 	list_add_tail(&xprt->xpt_ready, &pool->sp_sockets);
 	spin_unlock_bh(&pool->sp_lock);
 
-	/* find a thread for this xprt */
-	rcu_read_lock();
-	list_for_each_entry_rcu(rqstp, &pool->sp_all_threads, rq_all) {
-		if (test_and_set_bit(RQ_BUSY, &rqstp->rq_flags))
-			continue;
-		percpu_counter_inc(&pool->sp_threads_woken);
-		rqstp->rq_qtime = ktime_get();
-		wake_up_process(rqstp->rq_task);
-		goto out_unlock;
-	}
-	set_bit(SP_CONGESTED, &pool->sp_flags);
-	rqstp = NULL;
-out_unlock:
-	rcu_read_unlock();
+	svc_pool_wake_idle_thread(pool);
 }
 EXPORT_SYMBOL_GPL(svc_xprt_enqueue);
 
@@ -581,7 +567,10 @@ static void svc_xprt_release(struct svc_rqst *rqstp)
 	svc_xprt_put(xprt);
 }
 
-/*
+/**
+ * svc_wake_up - Wake up a service thread for non-transport work
+ * @serv: RPC service
+ *
  * Some svc_serv's will have occasional work to do, even when a xprt is not
  * waiting to be serviced. This function is there to "kick" a task in one of
  * those services so that it can wake up and do that work. Note that we only
@@ -590,27 +579,10 @@ static void svc_xprt_release(struct svc_rqst *rqstp)
  */
 void svc_wake_up(struct svc_serv *serv)
 {
-	struct svc_rqst	*rqstp;
-	struct svc_pool *pool;
-
-	pool = &serv->sv_pools[0];
-
-	rcu_read_lock();
-	list_for_each_entry_rcu(rqstp, &pool->sp_all_threads, rq_all) {
-		/* skip any that aren't queued */
-		if (test_bit(RQ_BUSY, &rqstp->rq_flags))
-			continue;
-		rcu_read_unlock();
-		wake_up_process(rqstp->rq_task);
-		trace_svc_wake_up(rqstp->rq_task->pid);
-		return;
-	}
-	rcu_read_unlock();
+	struct svc_pool *pool = &serv->sv_pools[0];
 
-	/* No free entries available */
-	set_bit(SP_TASK_PENDING, &pool->sp_flags);
-	smp_wmb();
-	trace_svc_wake_up(0);
+	if (!svc_pool_wake_idle_thread(pool))
+		set_bit(SP_TASK_PENDING, &pool->sp_flags);
 }
 EXPORT_SYMBOL_GPL(svc_wake_up);