BUG/MEDIUM: task/h2: add an idempotent task removal fucntion

Previous commit 3ea351368 ("BUG/MEDIUM: h2: Remove the tasklet from the
task list if unsubscribing.") uncovered an issue which needs to be
addressed in the scheduler's API. The function task_remove_from_task_list()
was initially designed to remove a task from the running tasklet list from
within the scheduler, and had to be used in h2 to abort pending I/O events.
However this function was not designed to be idempotent, occasionally
causing a double removal from the tasklet list, with the second doing
nothing but affecting the apparent tasks count and making haproxy use
100% CPU on some tests consisting in stopping the client during some
transfers. The h2_unsubscribe() function can sometimes be called upon
stream exit after an error where the tasklet was possibly already
removed, so it.

This patch does 2 things :
  - it renames task_remove_from_task_list() to
    __task_remove_from_tasklet_list() to discourage users from calling
    it. Also note the fix in the naming since it's a tasklet list and
    not a task list. This function is still uesd from the scheduler.
  - it adds a new, idempotent, task_remove_from_tasklet_list() function
    which does nothing if the task is already not in the tasklet list.

This patch will need to be backported where the commit above is backported.

include/proto/task.h

@@ -271,7 +271,10 @@ static inline void task_insert_into_tasklet_list(struct task *t)
 	LIST_ADDQ(&task_per_thread[tid].task_list, &tl->list);
 }
 
-static inline void task_remove_from_task_list(struct task *t)
+/* remove the task from the tasklet list. The task MUST already be there. If
+ * unsure, use task_remove_from_task_list() instead.
+ */
+static inline void __task_remove_from_tasklet_list(struct task *t)
 {
 	LIST_DEL_INIT(&((struct tasklet *)t)->list);
 	task_per_thread[tid].task_list_size--;
@@ -280,6 +283,12 @@ static inline void task_remove_from_task_list(struct task *t)
 	_HA_ATOMIC_SUB(&tasks_run_queue, 1);
 }
 
+static inline void task_remove_from_tasklet_list(struct task *t)
+{
+	if (likely(!LIST_ISEMPTY(&((struct tasklet *)t)->list)))
+		__task_remove_from_tasklet_list(t);
+}
+
 /*
  * Unlinks the task and adjusts run queue stats.
  * A pointer to the task itself is returned.

src/mux_h2.c

@@ -5182,7 +5182,7 @@ static int h2_unsubscribe(struct conn_stream *cs, int event_type, void *param)
 		sw = param;
 		if (h2s->send_wait == sw) {
 			sw->events &= ~SUB_CALL_UNSUBSCRIBE;
-			task_remove_from_task_list((struct task *)h2s->send_wait->task);
+			task_remove_from_tasklet_list((struct task *)h2s->send_wait->task);
 			h2s->send_wait = NULL;
 			LIST_DEL(&h2s->list);
 			LIST_INIT(&h2s->list);
@@ -5270,7 +5270,7 @@ static void h2_stop_senders(struct h2c *h2c)
 
 	list_for_each_entry_safe(h2s, h2s_back, &h2c->sending_list, sending_list) {
 		LIST_DEL_INIT(&h2s->sending_list);
-		task_remove_from_task_list((struct task *)h2s->send_wait->task);
+		task_remove_from_tasklet_list((struct task *)h2s->send_wait->task);
 		h2s->send_wait->events |= SUB_RETRY_SEND;
 		h2s->send_wait->events &= ~SUB_CALL_UNSUBSCRIBE;
 	}

src/task.c

@@ -416,7 +416,7 @@ void process_runnable_tasks()
 		t = (struct task *)LIST_ELEM(task_per_thread[tid].task_list.n, struct tasklet *, list);
 		state = _HA_ATOMIC_XCHG(&t->state, TASK_RUNNING);
 		__ha_barrier_atomic_store();
-		task_remove_from_task_list(t);
+		__task_remove_from_tasklet_list(t);
 
 		ctx = t->context;
 		process = t->process;