SUNRPC: change sp_nrthreads to atomic_t

Using an atomic_t avoids the need to take a spinlock (which can soon be
removed).

Choosing a thread to kill needs to be careful as we cannot set the "die
now" bit atomically with the test on the count.  Instead we temporarily
increase the count.

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

fs/nfsd/nfssvc.c

@@ -713,14 +713,13 @@ int nfsd_nrpools(struct net *net)
 
 int nfsd_get_nrthreads(int n, int *nthreads, struct net *net)
 {
-	int i = 0;
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+	struct svc_serv *serv = nn->nfsd_serv;
+	int i;
 
-	if (nn->nfsd_serv != NULL) {
-		for (i = 0; i < nn->nfsd_serv->sv_nrpools && i < n; i++)
-			nthreads[i] = nn->nfsd_serv->sv_pools[i].sp_nrthreads;
-	}
-
+	if (serv)
+		for (i = 0; i < serv->sv_nrpools && i < n; i++)
+			nthreads[i] = atomic_read(&serv->sv_pools[i].sp_nrthreads);
 	return 0;
 }
 

include/linux/sunrpc/svc.h

@@ -36,7 +36,7 @@ struct svc_pool {
 	unsigned int		sp_id;	    	/* pool id; also node id on NUMA */
 	spinlock_t		sp_lock;	/* protects all fields */
 	struct lwq		sp_xprts;	/* pending transports */
-	unsigned int		sp_nrthreads;	/* # of threads in pool */
+	atomic_t		sp_nrthreads;	/* # of threads in pool */
 	struct list_head	sp_all_threads;	/* all server threads */
 	struct llist_head	sp_idle_threads; /* idle server threads */
 

net/sunrpc/svc.c

@@ -681,8 +681,8 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 	serv->sv_nrthreads += 1;
 	spin_unlock_bh(&serv->sv_lock);
 
+	atomic_inc(&pool->sp_nrthreads);
 	spin_lock_bh(&pool->sp_lock);
-	pool->sp_nrthreads++;
 	list_add_rcu(&rqstp->rq_all, &pool->sp_all_threads);
 	spin_unlock_bh(&pool->sp_lock);
 	return rqstp;
@@ -727,32 +727,37 @@ svc_pool_next(struct svc_serv *serv, struct svc_pool *pool, unsigned int *state)
 }
 
 static struct svc_pool *
-svc_pool_victim(struct svc_serv *serv, struct svc_pool *pool, unsigned int *state)
+svc_pool_victim(struct svc_serv *serv, struct svc_pool *target_pool,
+		unsigned int *state)
 {
+	struct svc_pool *pool;
 	unsigned int i;
 
+retry:
+	pool = target_pool;
+
 	if (pool != NULL) {
-		spin_lock_bh(&pool->sp_lock);
-		if (pool->sp_nrthreads)
+		if (atomic_inc_not_zero(&pool->sp_nrthreads))
 			goto found_pool;
-		spin_unlock_bh(&pool->sp_lock);
 		return NULL;
 	} else {
 		for (i = 0; i < serv->sv_nrpools; i++) {
 			pool = &serv->sv_pools[--(*state) % serv->sv_nrpools];
-			spin_lock_bh(&pool->sp_lock);
-			if (pool->sp_nrthreads)
+			if (atomic_inc_not_zero(&pool->sp_nrthreads))
 				goto found_pool;
-			spin_unlock_bh(&pool->sp_lock);
 		}
 		return NULL;
 	}
 
 found_pool:
 	set_bit(SP_VICTIM_REMAINS, &pool->sp_flags);
 	set_bit(SP_NEED_VICTIM, &pool->sp_flags);
-	spin_unlock_bh(&pool->sp_lock);
-	return pool;
+	if (!atomic_dec_and_test(&pool->sp_nrthreads))
+		return pool;
+	/* Nothing left in this pool any more */
+	clear_bit(SP_NEED_VICTIM, &pool->sp_flags);
+	clear_bit(SP_VICTIM_REMAINS, &pool->sp_flags);
+	goto retry;
 }
 
 static int
@@ -828,13 +833,10 @@ svc_stop_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 int
 svc_set_num_threads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 {
-	if (pool == NULL) {
+	if (!pool)
 		nrservs -= serv->sv_nrthreads;
-	} else {
-		spin_lock_bh(&pool->sp_lock);
-		nrservs -= pool->sp_nrthreads;
-		spin_unlock_bh(&pool->sp_lock);
-	}
+	else
+		nrservs -= atomic_read(&pool->sp_nrthreads);
 
 	if (nrservs > 0)
 		return svc_start_kthreads(serv, pool, nrservs);
@@ -921,10 +923,11 @@ svc_exit_thread(struct svc_rqst *rqstp)
 	struct svc_pool	*pool = rqstp->rq_pool;
 
 	spin_lock_bh(&pool->sp_lock);
-	pool->sp_nrthreads--;
 	list_del_rcu(&rqstp->rq_all);
 	spin_unlock_bh(&pool->sp_lock);
 
+	atomic_dec(&pool->sp_nrthreads);
+
 	spin_lock_bh(&serv->sv_lock);
 	serv->sv_nrthreads -= 1;
 	spin_unlock_bh(&serv->sv_lock);