diff --git a/src/base.c b/src/base.c
index 7502a657..dd4b109d 100644
--- a/src/base.c
+++ b/src/base.c
@@ -199,8 +199,14 @@ base_extent_alloc(tsdn_t *tsdn, base_t *base, size_t size, size_t alignment) {
 	malloc_mutex_assert_owner(tsdn, &base->mtx);
 
 	extent_hooks_t *extent_hooks = base_extent_hooks_get(base);
+	/*
+	 * Drop mutex during base_block_alloc(), because an extent hook will be
+	 * called.
+	 */
+	malloc_mutex_unlock(tsdn, &base->mtx);
 	base_block_t *block = base_block_alloc(extent_hooks, base_ind_get(base),
 	    &base->pind_last, &base->extent_sn_next, size, alignment);
+	malloc_mutex_lock(tsdn, &base->mtx);
 	if (block == NULL) {
 		return NULL;
 	}