SUNRPC: change service idle list to be an llist

With an llist we don't need to take a lock to add a thread to the list,
though we still need a lock to remove it.  That will go in the next
patch.

Unlike double-linked lists, a thread cannot reliably remove itself from
the list.  Only the first thread can be removed, and that can change
asynchronously.  So some care is needed.

We already check if there is pending work to do, so we are unlikely to
add ourselves to the idle list and then want to remove ourselves again.

If we DO find something needs to be done after adding ourselves to the
list, we simply wake up the first thread on the list.  If that was us,
we successfully removed ourselves and can continue.  If it was some
other thread, they will do the work that needs to be done.  We can
safely sleep until woken.

We also remove the test on freezing() from rqst_should_sleep().  Instead
we set TASK_FREEZABLE before scheduling.  This makes is safe to
schedule() when a freeze is pending.  As we now loop waiting to be
removed from the idle queue, this is a cleaner way to handle freezing.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

1 files changed, 7 insertions, 7 deletions

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index db4674211f36..54ae6a569f6a 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -510,7 +510,7 @@ __svc_create(struct svc_program *prog, unsigned int bufsize, int npools,
 		pool->sp_id = i;
 		INIT_LIST_HEAD(&pool->sp_sockets);
 		INIT_LIST_HEAD(&pool->sp_all_threads);
-		INIT_LIST_HEAD(&pool->sp_idle_threads);
+		init_llist_head(&pool->sp_idle_threads);
 		spin_lock_init(&pool->sp_lock);
 
 		percpu_counter_init(&pool->sp_messages_arrived, 0, GFP_KERNEL);
@@ -642,7 +642,7 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
 
 	folio_batch_init(&rqstp->rq_fbatch);
 
-	svc_thread_set_busy(rqstp);
+	init_llist_node(&rqstp->rq_idle);
 	rqstp->rq_server = serv;
 	rqstp->rq_pool = pool;
 
@@ -701,15 +701,15 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 void svc_pool_wake_idle_thread(struct svc_pool *pool)
 {
 	struct svc_rqst	*rqstp;
+	struct llist_node *ln;
 
 	rcu_read_lock();
 	spin_lock_bh(&pool->sp_lock);
-	rqstp = list_first_entry_or_null(&pool->sp_idle_threads,
-					 struct svc_rqst, rq_idle);
-	if (rqstp)
-		list_del_init(&rqstp->rq_idle);
+	ln = llist_del_first_init(&pool->sp_idle_threads);
 	spin_unlock_bh(&pool->sp_lock);
-	if (rqstp) {
+	if (ln) {
+		rqstp = llist_entry(ln, struct svc_rqst, rq_idle);
+
 		WRITE_ONCE(rqstp->rq_qtime, ktime_get());
 		wake_up_process(rqstp->rq_task);
 		rcu_read_unlock();