diff --git a/lib/core-net/close.c b/lib/core-net/close.c
index e8f756788..be113b4ef 100644
--- a/lib/core-net/close.c
+++ b/lib/core-net/close.c
@@ -275,6 +275,9 @@ __lws_close_free_wsi(struct lws *wsi, enum lws_close_status reason,
 
 	context = wsi->a.context;
 	pt = &context->pt[(int)wsi->tsi];
+
+	lws_pt_assert_lock_held(pt);
+
 	lws_stats_bump(pt, LWSSTATS_C_API_CLOSE, 1);
 
 #if defined(LWS_WITH_CLIENT)
diff --git a/lib/core-net/pollfd.c b/lib/core-net/pollfd.c
index eaeda9420..894d7772a 100644
--- a/lib/core-net/pollfd.c
+++ b/lib/core-net/pollfd.c
@@ -271,6 +271,8 @@ __insert_wsi_socket_into_fds(struct lws_context *context, struct lws *wsi)
 
 //	__dump_fds(pt, "pre insert");
 
+	lws_pt_assert_lock_held(pt);
+
 	lwsl_debug("%s: %p: tsi=%d, sock=%d, pos-in-fds=%d\n",
 		  __func__, wsi, wsi->tsi, wsi->desc.sockfd, pt->fds_count);
 
@@ -352,6 +354,8 @@ __remove_wsi_socket_from_fds(struct lws *wsi)
 	struct lws *end_wsi;
 	int v, m, ret = 0;
 
+	lws_pt_assert_lock_held(pt);
+
 //	__dump_fds(pt, "pre remove");
 
 #if !defined(_WIN32)
diff --git a/lib/core-net/private-lib-core-net.h b/lib/core-net/private-lib-core-net.h
index 05ca1e700..ea785c509 100644
--- a/lib/core-net/private-lib-core-net.h
+++ b/lib/core-net/private-lib-core-net.h
@@ -999,6 +999,7 @@ lws_pt_mutex_destroy(struct lws_context_per_thread *pt)
 
 #define lws_pt_lock(pt, reason) lws_mutex_refcount_lock(&pt->mr, reason)
 #define lws_pt_unlock(pt) lws_mutex_refcount_unlock(&pt->mr)
+#define lws_pt_assert_lock_held(pt) lws_mutex_refcount_assert_held(&pt->mr)
 
 static LWS_INLINE void
 lws_pt_stats_lock(struct lws_context_per_thread *pt)
diff --git a/lib/core-net/vhost.c b/lib/core-net/vhost.c
index 09cb979a3..5e01967d6 100644
--- a/lib/core-net/vhost.c
+++ b/lib/core-net/vhost.c
@@ -908,6 +908,7 @@ lws_cancel_service(struct lws_context *context)
 int
 lws_create_event_pipes(struct lws_context *context)
 {
+	struct lws_context_per_thread *pt;
 	size_t s = sizeof(struct lws);
 	struct lws *wsi;
 	int n;
@@ -923,7 +924,9 @@ lws_create_event_pipes(struct lws_context *context)
 	n = 0;
 	{
 #endif
-		if (context->pt[n].pipe_wsi)
+		pt = &context->pt[n];
+
+		if (pt->pipe_wsi)
 			return 0;
 
 #if defined(LWS_WITH_EVENT_LIBS)
@@ -948,6 +951,8 @@ lws_create_event_pipes(struct lws_context *context)
 		context->pt[n].pipe_wsi = wsi;
 		context->count_wsi_allocated++;
 
+		lws_pt_lock(pt, __func__); /* -------------- pt { */
+
 		if (!lws_plat_pipe_create(wsi)) {
 			/*
 			 * platform code returns 0 if it actually created pipes
@@ -963,14 +968,21 @@ lws_create_event_pipes(struct lws_context *context)
 
 			if (context->event_loop_ops->sock_accept)
 				if (context->event_loop_ops->sock_accept(wsi))
-					return 1;
+					goto bail;
 
 			if (__insert_wsi_socket_into_fds(context, wsi))
-				return 1;
+				goto bail;
 		}
+
+		lws_pt_unlock(pt);
 	}
 
 	return 0;
+
+bail:
+	lws_pt_unlock(pt);
+
+	return 1;
 }
 
 void
diff --git a/lib/core-net/wsi-timeout.c b/lib/core-net/wsi-timeout.c
index f3610a5b2..f6b9fac75 100644
--- a/lib/core-net/wsi-timeout.c
+++ b/lib/core-net/wsi-timeout.c
@@ -118,7 +118,9 @@ lws_sul_wsitimeout_cb(lws_sorted_usec_list_t *sul)
 			(void *)"Timed out waiting SSL", 21);
 #endif
 
+	lws_pt_lock(pt, __func__);
 	__lws_close_free_wsi(wsi, LWS_CLOSE_STATUS_NOSTATUS, "timeout");
+	lws_pt_unlock(pt);
 }
 
 void
diff --git a/lib/core/context.c b/lib/core/context.c
index 85f77b0f4..d25e9161b 100644
--- a/lib/core/context.c
+++ b/lib/core/context.c
@@ -1540,8 +1540,10 @@ lws_pt_destroy(struct lws_context_per_thread *pt)
 	}
 	vpt->foreign_pfd_list = NULL;
 
+	lws_pt_lock(pt, __func__);
 	if (pt->pipe_wsi)
 		lws_destroy_event_pipe(pt->pipe_wsi);
+	lws_pt_unlock(pt);
 	pt->pipe_wsi = NULL;
 
 	while (pt->fds_count) {
diff --git a/lib/core/libwebsockets.c b/lib/core/libwebsockets.c
index e3b7c954d..a44e6d5f5 100644
--- a/lib/core/libwebsockets.c
+++ b/lib/core/libwebsockets.c
@@ -1194,6 +1194,12 @@ lws_mutex_refcount_unlock(struct lws_mutex_refcount *mr)
 	pthread_mutex_unlock(&mr->lock);
 }
 
+void
+lws_mutex_refcount_assert_held(struct lws_mutex_refcount *mr)
+{
+	assert(mr->lock_owner == pthread_self() && mr->lock_depth);
+}
+
 #endif /* SMP */
 
 
diff --git a/lib/core/private-lib-core.h b/lib/core/private-lib-core.h
index ca82651a9..b4dcf7050 100644
--- a/lib/core/private-lib-core.h
+++ b/lib/core/private-lib-core.h
@@ -250,6 +250,9 @@ lws_mutex_refcount_destroy(struct lws_mutex_refcount *mr);
 void
 lws_mutex_refcount_lock(struct lws_mutex_refcount *mr, const char *reason);
 
+void
+lws_mutex_refcount_assert_held(struct lws_mutex_refcount *mr);
+
 void
 lws_mutex_refcount_unlock(struct lws_mutex_refcount *mr);
 #endif
@@ -644,6 +647,7 @@ lws_vhost_unlock(struct lws_vhost *vhost)
 #define lws_pt_mutex_init(_a) (void)(_a)
 #define lws_pt_mutex_destroy(_a) (void)(_a)
 #define lws_pt_lock(_a, b) (void)(_a)
+#define lws_pt_assert_lock_held(_a) (void)(_a)
 #define lws_pt_unlock(_a) (void)(_a)
 #define lws_context_lock(_a, _b) (void)(_a)
 #define lws_context_unlock(_a) (void)(_a)
diff --git a/lib/roles/http/server/server.c b/lib/roles/http/server/server.c
index 01a6b38bc..c8683fc05 100644
--- a/lib/roles/http/server/server.c
+++ b/lib/roles/http/server/server.c
@@ -52,6 +52,7 @@ int
 _lws_vhost_init_server(const struct lws_context_creation_info *info,
 		       struct lws_vhost *vhost)
 {
+	struct lws_context_per_thread *pt;
 	int n, opt = 1, limit = 1;
 	lws_sockfd_type sockfd;
 	struct lws_vhost *vh;
@@ -309,13 +310,18 @@ done_list:
 		if (wsi->a.context->event_loop_ops->init_vhost_listen_wsi)
 			wsi->a.context->event_loop_ops->init_vhost_listen_wsi(wsi);
 
+		pt = &vhost->context->pt[m];
+		lws_pt_lock(pt, __func__);
+
 		if (__insert_wsi_socket_into_fds(vhost->context, wsi)) {
 			lwsl_notice("inserting wsi socket into fds failed\n");
+			lws_pt_unlock(pt);
 			goto bail;
 		}
 
 		vhost->context->count_wsi_allocated++;
 		vhost->lserv_wsi = wsi;
+		lws_pt_unlock(pt);
 
 		n = listen(wsi->desc.sockfd, LWS_SOMAXCONN);
 		if (n < 0) {