From f461ab428e100d9513a9beb50d643fa43936bc8a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Markus=20M=C3=A4kel=C3=A4?= <markus.makela@mariadb.com>
Date: Fri, 19 Oct 2018 13:44:21 +0300
Subject: [PATCH] Fix backend DCB writeq throttling

The callbacks iterated over all threads when only the local ones must be
iterated. This prevents a deadlock from occurring when multiple threads
start throttling at the same time.

Also fixed the gwbuf_append debug assertion.
---
 server/core/buffer.cc | 5 +++--
 server/core/dcb.cc    | 4 ++--
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/server/core/buffer.cc b/server/core/buffer.cc
index a40fc93c2..beb1832a0 100644
--- a/server/core/buffer.cc
+++ b/server/core/buffer.cc
@@ -450,6 +450,9 @@ int gwbuf_compare(const GWBUF* lhs, const GWBUF* rhs)
 
 GWBUF* gwbuf_append(GWBUF* head, GWBUF* tail)
 {
+    mxb_assert(!head || head->owner == RoutingWorker::get_current_id());
+    mxb_assert(!tail || tail->owner == RoutingWorker::get_current_id());
+
     if (!head)
     {
         return tail;
@@ -459,8 +462,6 @@ GWBUF* gwbuf_append(GWBUF* head, GWBUF* tail)
         return head;
     }
 
-    mxb_assert(head->owner == RoutingWorker::get_current_id()
-               && tail->owner == RoutingWorker::get_current_id());
     head->tail->next = tail;
     head->tail = tail->tail;
 
diff --git a/server/core/dcb.cc b/server/core/dcb.cc
index 24c26baa9..0838ec4f1 100644
--- a/server/core/dcb.cc
+++ b/server/core/dcb.cc
@@ -3768,11 +3768,11 @@ static int downstream_throttle_callback(DCB* dcb, DCB_REASON reason, void* userd
 {
     if (reason == DCB_REASON_HIGH_WATER)
     {
-        dcb_foreach(backend_dcb_remove_func, dcb->session);
+        dcb_foreach_local(backend_dcb_remove_func, dcb->session);
     }
     else if (reason == DCB_REASON_LOW_WATER)
     {
-        dcb_foreach(backend_dcb_add_func, dcb->session);
+        dcb_foreach_local(backend_dcb_add_func, dcb->session);
     }
 
     return 0;