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>

1 files changed, 34 insertions, 0 deletions

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 7873a3ff24a8..ee9d55b2f275 100644
--- a/net/sunrpc/svc.c
+++ b/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
  */