diff --git a/sources/route_pool.h b/sources/route_pool.h
index 2642f7ae..ee6bc7b2 100644
--- a/sources/route_pool.h
+++ b/sources/route_pool.h
@@ -131,11 +131,12 @@ od_route_pool_stat(od_route_pool_t *pool,
 		od_stat_t avg;
 		od_stat_init(&avg);
 		if (route->stats.enable_quantiles) {
+			uint8_t current_tdigest = route->stats.current_tdigest;
 			next_tdigest =
-			  (route->stats.current_tdigest + 1) % QUANTILES_WINDOW;
+					(current_tdigest + 1) % QUANTILES_WINDOW;
 			td_reset(
-			  route->stats.transaction_hgram[route->stats.current_tdigest]);
-			td_reset(route->stats.query_hgram[route->stats.current_tdigest]);
+			  route->stats.transaction_hgram[current_tdigest]);
+			td_reset(route->stats.query_hgram[current_tdigest]);
 			route->stats.current_tdigest = next_tdigest;
 		}
 
diff --git a/sources/tdigest.c b/sources/tdigest.c
index c6c05b83..d9a54ad0 100644
--- a/sources/tdigest.c
+++ b/sources/tdigest.c
@@ -5,6 +5,7 @@
 #include <assert.h>
 
 #include "tdigest.h"
+#include "sleep_lock.h"
 
 typedef struct node
 {
@@ -28,6 +29,8 @@ struct td_histogram
 	double merged_count;
 	double unmerged_count;
 
+	mm_sleeplock_t lock;
+
 	node_t nodes[0];
 };
 
@@ -136,11 +139,13 @@ td_merge(td_histogram_t *into, td_histogram_t *from)
 void
 td_reset(td_histogram_t *h)
 {
+	mm_sleeplock_lock(&h->lock);
 	bzero((void *)(&h->nodes[0]), sizeof(node_t) * h->cap);
 	h->merged_nodes   = 0;
 	h->merged_count   = 0;
 	h->unmerged_nodes = 0;
 	h->unmerged_count = 0;
+	mm_sleeplock_unlock(&h->lock);
 }
 
 void
@@ -300,6 +305,7 @@ td_trimmed_mean(td_histogram_t *h, double lo, double hi)
 void
 td_add(td_histogram_t *h, double mean, double count)
 {
+	mm_sleeplock_lock(&h->lock);
 	if (should_merge(h)) {
 		merge(h);
 	}
@@ -309,6 +315,7 @@ td_add(td_histogram_t *h, double mean, double count)
 	};
 	h->unmerged_nodes++;
 	h->unmerged_count += count;
+	mm_sleeplock_unlock(&h->lock);
 }
 
 static int