sunrpc/cache: allow threads to block while waiting for cache update.

The current practice of waiting for cache updates by queueing the
whole request to be retried has (at least) two problems.

1/ With NFSv4, requests can be quite complex and re-trying a whole
  request when a later part fails should only be a last-resort, not a
  normal practice.

2/ Large requests, and in particular any 'write' request, will not be
  queued by the current code and doing so would be undesirable.

In many cases only a very sort wait is needed before the cache gets
valid data.

So, providing the underlying transport permits it by setting
 ->thread_wait,
arrange to wait briefly for an upcall to be completed (as reflected in
the clearing of CACHE_PENDING).
If the short wait was not long enough and CACHE_PENDING is still set,
fall back on the old approach.

The 'thread_wait' value is set to 5 seconds when there are spare
threads, and 1 second when there are no spare threads.

These values are probably much higher than needed, but will ensure
some forward progress.

Note that as we only request an update for a non-valid item, and as
non-valid items are updated in place it is extremely unlikely that
cache_check will return -ETIMEDOUT.  Normally cache_defer_req will
sleep for a short while and then find that the item is_valid.

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

1 files changed, 58 insertions, 1 deletions

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index 8dc121955fd..2c5297f245b 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -509,10 +509,22 @@ static LIST_HEAD(cache_defer_list);
 static struct list_head cache_defer_hash[DFR_HASHSIZE];
 static int cache_defer_cnt;
 
+struct thread_deferred_req {
+	struct cache_deferred_req handle;
+	struct completion completion;
+};
+static void cache_restart_thread(struct cache_deferred_req *dreq, int too_many)
+{
+	struct thread_deferred_req *dr =
+		container_of(dreq, struct thread_deferred_req, handle);
+	complete(&dr->completion);
+}
+
 static int cache_defer_req(struct cache_req *req, struct cache_head *item)
 {
 	struct cache_deferred_req *dreq, *discard;
 	int hash = DFR_HASH(item);
+	struct thread_deferred_req sleeper;
 
 	if (cache_defer_cnt >= DFR_MAX) {
 		/* too much in the cache, randomly drop this one,
@@ -521,7 +533,15 @@ static int cache_defer_req(struct cache_req *req, struct cache_head *item)
 		if (net_random()&1)
 			return -ENOMEM;
 	}
-	dreq = req->defer(req);
+	if (req->thread_wait) {
+		dreq = &sleeper.handle;
+		sleeper.completion =
+			COMPLETION_INITIALIZER_ONSTACK(sleeper.completion);
+		dreq->revisit = cache_restart_thread;
+	} else
+		dreq = req->defer(req);
+
+ retry:
 	if (dreq == NULL)
 		return -ENOMEM;
 
@@ -555,6 +575,43 @@ static int cache_defer_req(struct cache_req *req, struct cache_head *item)
 		cache_revisit_request(item);
 		return -EAGAIN;
 	}
+
+	if (dreq == &sleeper.handle) {
+		if (wait_for_completion_interruptible_timeout(
+			    &sleeper.completion, req->thread_wait) <= 0) {
+			/* The completion wasn't completed, so we need
+			 * to clean up
+			 */
+			spin_lock(&cache_defer_lock);
+			if (!list_empty(&sleeper.handle.hash)) {
+				list_del_init(&sleeper.handle.recent);
+				list_del_init(&sleeper.handle.hash);
+				cache_defer_cnt--;
+				spin_unlock(&cache_defer_lock);
+			} else {
+				/* cache_revisit_request already removed
+				 * this from the hash table, but hasn't
+				 * called ->revisit yet.  It will very soon
+				 * and we need to wait for it.
+				 */
+				spin_unlock(&cache_defer_lock);
+				wait_for_completion(&sleeper.completion);
+			}
+		}
+		if (test_bit(CACHE_PENDING, &item->flags)) {
+			/* item is still pending, try request
+			 * deferral
+			 */
+			dreq = req->defer(req);
+			goto retry;
+		}
+		/* only return success if we actually deferred the
+		 * request.  In this case we waited until it was
+		 * answered so no deferral has happened - rather
+		 * an answer already exists.
+		 */
+		return -EEXIST;
+	}
 	return 0;
 }