sunrpc/cache: centralise handling of size limit on deferred list.

We limit the number of 'defer' requests to DFR_MAX.

The imposition of this limit is spread about a bit - sometime we don't
add new things to the list, sometimes we remove old things.

Also it is currently applied to requests which we are 'waiting' for
rather than 'deferring'.  This doesn't seem ideal as 'waiting'
requests are naturally limited by the number of threads.

So gather the DFR_MAX handling code to one place and only apply it to
requests that are actually being deferred.

This means that not all 'cache_deferred_req' structures go on the
'cache_defer_list, so we need to be careful when adding and removing
things.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/cache.c

@@ -513,41 +513,39 @@ static int cache_defer_cnt;
 
 static void __unhash_deferred_req(struct cache_deferred_req *dreq)
 {
-	list_del_init(&dreq->recent);
 	hlist_del_init(&dreq->hash);
-	cache_defer_cnt--;
+	if (!list_empty(&dreq->recent)) {
+		list_del_init(&dreq->recent);
+		cache_defer_cnt--;
+	}
 }
 
 static void __hash_deferred_req(struct cache_deferred_req *dreq, struct cache_head *item)
 {
 	int hash = DFR_HASH(item);
 
-	list_add(&dreq->recent, &cache_defer_list);
+	INIT_LIST_HEAD(&dreq->recent);
 	hlist_add_head(&dreq->hash, &cache_defer_hash[hash]);
 }
 
-static void setup_deferral(struct cache_deferred_req *dreq, struct cache_head *item)
+static void setup_deferral(struct cache_deferred_req *dreq,
+			   struct cache_head *item,
+			   int count_me)
 {
-	struct cache_deferred_req *discard;
 
 	dreq->item = item;
 
 	spin_lock(&cache_defer_lock);
 
 	__hash_deferred_req(dreq, item);
 
-	/* it is in, now maybe clean up */
-	discard = NULL;
-	if (++cache_defer_cnt > DFR_MAX) {
-		discard = list_entry(cache_defer_list.prev,
-				     struct cache_deferred_req, recent);
-		__unhash_deferred_req(discard);
+	if (count_me) {
+		cache_defer_cnt++;
+		list_add(&dreq->recent, &cache_defer_list);
 	}
+
 	spin_unlock(&cache_defer_lock);
 
-	if (discard)
-		/* there was one too many */
-		discard->revisit(discard, 1);
 }
 
 struct thread_deferred_req {
@@ -570,7 +568,7 @@ static void cache_wait_req(struct cache_req *req, struct cache_head *item)
 	sleeper.completion = COMPLETION_INITIALIZER_ONSTACK(sleeper.completion);
 	dreq->revisit = cache_restart_thread;
 
-	setup_deferral(dreq, item);
+	setup_deferral(dreq, item, 0);
 
 	if (!test_bit(CACHE_PENDING, &item->flags) ||
 	    wait_for_completion_interruptible_timeout(
@@ -594,17 +592,36 @@ static void cache_wait_req(struct cache_req *req, struct cache_head *item)
 	}
 }
 
-static void cache_defer_req(struct cache_req *req, struct cache_head *item)
+static void cache_limit_defers(void)
 {
-	struct cache_deferred_req *dreq;
+	/* Make sure we haven't exceed the limit of allowed deferred
+	 * requests.
+	 */
+	struct cache_deferred_req *discard = NULL;
 
-	if (cache_defer_cnt >= DFR_MAX)
-		/* too much in the cache, randomly drop this one,
-		 * or continue and drop the oldest
-		 */
-		if (net_random()&1)
-			return;
+	if (cache_defer_cnt <= DFR_MAX)
+		return;
+
+	spin_lock(&cache_defer_lock);
 
+	/* Consider removing either the first or the last */
+	if (cache_defer_cnt > DFR_MAX) {
+		if (net_random() & 1)
+			discard = list_entry(cache_defer_list.next,
+					     struct cache_deferred_req, recent);
+		else
+			discard = list_entry(cache_defer_list.prev,
+					     struct cache_deferred_req, recent);
+		__unhash_deferred_req(discard);
+	}
+	spin_unlock(&cache_defer_lock);
+	if (discard)
+		discard->revisit(discard, 1);
+}
+
+static void cache_defer_req(struct cache_req *req, struct cache_head *item)
+{
+	struct cache_deferred_req *dreq;
 
 	if (req->thread_wait) {
 		cache_wait_req(req, item);
@@ -614,12 +631,14 @@ static void cache_defer_req(struct cache_req *req, struct cache_head *item)
 	dreq = req->defer(req);
 	if (dreq == NULL)
 		return;
-	setup_deferral(dreq, item);
+	setup_deferral(dreq, item, 1);
 	if (!test_bit(CACHE_PENDING, &item->flags))
 		/* Bit could have been cleared before we managed to
 		 * set up the deferral, so need to revisit just in case
 		 */
 		cache_revisit_request(item);
+
+	cache_limit_defers();
 }
 
 static void cache_revisit_request(struct cache_head *item)