diff --git a/include/proto/task.h b/include/proto/task.h
index 8649a88..5c7aa6a 100644
--- a/include/proto/task.h
+++ b/include/proto/task.h
@@ -150,7 +150,13 @@
 	}
 }
 
-/* change the thread affinity of a task to <thread_mask> */
+/* change the thread affinity of a task to <thread_mask>.
+ * This may only be done from within the running task itself or during its
+ * initialization. It will unqueue and requeue the task from the wait queue
+ * if it was in it. This is safe against a concurrent task_queue() call because
+ * task_queue() itself will unlink again if needed after taking into account
+ * the new thread_mask.
+ */
 static inline void task_set_affinity(struct task *t, unsigned long thread_mask)
 {
 	if (unlikely(task_in_wq(t))) {
@@ -175,15 +181,15 @@
 }
 
 /* remove a task from its wait queue. It may either be the local wait queue if
- * the task is bound to a single thread (in which case there's no locking
- * involved) or the global queue, with locking.
+ * the task is bound to a single thread or the global queue. If the task uses a
+ * shared wait queue, the global wait queue lock is used.
  */
 static inline struct task *task_unlink_wq(struct task *t)
 {
 	unsigned long locked;
 
 	if (likely(task_in_wq(t))) {
-		locked = atleast2(t->thread_mask);
+		locked = t->state & TASK_SHARED_WQ;
 		if (locked)
 			HA_RWLOCK_WRLOCK(TASK_WQ_LOCK, &wq_lock);
 		__task_unlink_wq(t);
@@ -269,7 +275,8 @@
 /*
  * Initialize a new task. The bare minimum is performed (queue pointers and
  * state).  The task is returned. This function should not be used outside of
- * task_new().
+ * task_new(). If the thread mask contains more than one thread, TASK_SHARED_WQ
+ * is set.
  */
 static inline struct task *task_init(struct task *t, unsigned long thread_mask)
 {
@@ -277,6 +284,8 @@
 	t->rq.node.leaf_p = NULL;
 	t->state = TASK_SLEEPING;
 	t->thread_mask = thread_mask;
+	if (atleast2(thread_mask))
+		t->state |= TASK_SHARED_WQ;
 	t->nice = 0;
 	t->calls = 0;
 	t->call_date = 0;
@@ -381,9 +390,9 @@
 
 /* Place <task> into the wait queue, where it may already be. If the expiration
  * timer is infinite, do nothing and rely on wake_expired_task to clean up.
- * If the task is bound to a single thread, it's assumed to be bound to the
- * current thread's queue and is queued without locking. Otherwise it's queued
- * into the global wait queue, protected by locks.
+ * If the task uses a shared wait queue, it's queued into the global wait queue,
+ * protected by the global wq_lock, otherwise by it necessarily belongs to the
+ * current thread'sand is queued without locking.
  */
 static inline void task_queue(struct task *task)
 {
@@ -400,7 +409,7 @@
 		return;
 
 #ifdef USE_THREAD
-	if (atleast2(task->thread_mask)) {
+	if (task->state & TASK_SHARED_WQ) {
 		HA_RWLOCK_WRLOCK(TASK_WQ_LOCK, &wq_lock);
 		if (!task_in_wq(task) || tick_is_lt(task->expire, task->wq.key))
 			__task_queue(task, &timers);
@@ -408,6 +417,7 @@
 	} else
 #endif
 	{
+		BUG_ON((task->thread_mask & tid_bit) == 0); // should have TASK_SHARED_WQ
 		if (!task_in_wq(task) || tick_is_lt(task->expire, task->wq.key))
 			__task_queue(task, &task_per_thread[tid].timers);
 	}
@@ -424,7 +434,7 @@
 		return;
 
 #ifdef USE_THREAD
-	if (atleast2(task->thread_mask)) {
+	if (task->state & TASK_SHARED_WQ) {
 		/* FIXME: is it really needed to lock the WQ during the check ? */
 		HA_RWLOCK_WRLOCK(TASK_WQ_LOCK, &wq_lock);
 		if (task_in_wq(task))
@@ -437,6 +447,7 @@
 	} else
 #endif
 	{
+		BUG_ON((task->thread_mask & tid_bit) == 0); // should have TASK_SHARED_WQ
 		if (task_in_wq(task))
 			when = tick_first(when, task->expire);