BUG/MINOR: event_hdl: make event_hdl_subscribe thread-safe

List insertion in event_hdl_subscribe() was not thread-safe when dealing
with unique identifiers. Indeed, in this case the list insertion is
conditional (we check for a duplicate, then we insert). And while we're
using mt lists for this, the whole operation is not atomic: there is a
race between the check and the insertion.
This could lead to the same ID being registered multiple times with
concurrent calls to event_hdl_subscribe() on the same ID.

To fix this, we add 'insert_lock' dedicated lock in the subscription
list struct. The lock's cost is nearly 0 since it is only used when
registering identified subscriptions and the lock window is very short:
we only guard the duplicate check and the list insertion to make the
conditional insertion "atomic" within a given subscription list.
This is the only place where we need the lock: as soon as the item is
properly inserted we're out of trouble because all other operations on
the list are already thread-safe thanks to mt lists.

A new lock hint is introduced: LOCK_EHDL which is dedicated to event_hdl

The patch may seem quite large since we had to rework the logic around
the subscribe function and switch from simple mt_list to a dedicated
struct wrapping both the mt_list and the insert_lock for the
event_hdl_sub_list type.
(sizeof(event_hdl_sub_list) is now 24 instead of 16)

However, all the changes are internal: we don't break the API.

If 68e692da0 ("MINOR: event_hdl: add event handler base api")
is being backported, then this commit should be backported with it.

include/haproxy/event_hdl-t.h

@@ -24,7 +24,8 @@
 
 #include <stdint.h>
 
-#include <haproxy/list-t.h>
+#include <haproxy/api-t.h>
+#include <haproxy/thread-t.h>
 
 /* event data struct are defined as followed */
 struct event_hdl_cb_data_template {
@@ -72,8 +73,13 @@ struct event_hdl_sub_type
 	uint16_t subtype;
 };
 
-/* event_hdl_sub_list is an alias to mt_list (please use this for portability) */
-typedef struct mt_list event_hdl_sub_list;
+struct event_hdl_sub_list_head {
+	struct mt_list head;
+	__decl_thread(HA_SPINLOCK_T insert_lock);
+};
+
+/* event_hdl_sub_list is an alias (please use this for portability) */
+typedef struct event_hdl_sub_list_head event_hdl_sub_list;
 /* event_hdl_async_equeue is an alias to mt_list (please use this for portability) */
 typedef struct mt_list event_hdl_async_equeue;
 

include/haproxy/event_hdl.h

@@ -221,6 +221,14 @@ uint64_t event_hdl_id(const char *scope, const char *name);
  * If <sub_list> is not specified (equals NULL):
  * global subscription list (process wide) will be used.
  *
+ * For identified subscriptions (EVENT_HDL_ID_*), the function is safe against
+ * concurrent subscriptions attempts with the same ID: the ID will only be
+ * inserted once in the list and subsequent attempts will yield an error.
+ * However, trying to register the same ID multiple times is considered as
+ * an error (no specific error code is returned in this case) so the check should
+ * be performed by the caller if it is expected. (The caller must ensure that the ID
+ * is unique to prevent the error from being raised)
+ *
  * Returns 1 in case of success, 0 in case of failure (invalid argument / memory error)
  */
 int event_hdl_subscribe(event_hdl_sub_list *sub_list,
@@ -422,7 +430,8 @@ static inline struct event_hdl_async_event *event_hdl_async_equeue_pop(event_hdl
  */
 static inline void event_hdl_sub_list_init(event_hdl_sub_list *sub_list)
 {
-	MT_LIST_INIT(sub_list);
+	MT_LIST_INIT(&sub_list->head);
+	HA_SPIN_INIT(&sub_list->insert_lock);
 }
 
 /* use this function when you need to destroy <sub_list>

include/haproxy/thread.h

@@ -425,6 +425,7 @@ enum lock_label {
 	IDLE_CONNS_LOCK,
 	QUIC_LOCK,
 	OCSP_LOCK,
+	EHDL_LOCK,
 	OTHER_LOCK,
 	/* WT: make sure never to use these ones outside of development,
 	 * we need them for lock profiling!

src/event_hdl.c

@@ -43,14 +43,20 @@ DECLARE_STATIC_POOL(pool_head_sub_event, "ehdl_sub_e", sizeof(struct event_hdl_a
 DECLARE_STATIC_POOL(pool_head_sub_event_data, "ehdl_sub_ed", sizeof(struct event_hdl_async_event_data));
 DECLARE_STATIC_POOL(pool_head_sub_taskctx, "ehdl_sub_tctx", sizeof(struct event_hdl_async_task_default_ctx));
 
-/* global subscription list (implicit where NULL is used as sublist argument) */
-static struct mt_list global_event_hdl_sub_list = MT_LIST_HEAD_INIT(global_event_hdl_sub_list);
-
 /* TODO: will become a config tunable
  * ie: tune.events.max-async-notif-at-once
  */
 static int event_hdl_async_max_notif_at_once = 10;
 
+/* global subscription list (implicit where NULL is used as sublist argument) */
+static event_hdl_sub_list global_event_hdl_sub_list;
+
+static void event_hdl_init(void)
+{
+	/* initialize global subscription list */
+	event_hdl_sub_list_init(&global_event_hdl_sub_list);
+}
+
 /* general purpose hashing function when you want to compute
  * an ID based on <scope> x <name>
  * It is your responsibility to make sure <scope> is not used
@@ -372,10 +378,9 @@ static void event_hdl_unsubscribe_async(const struct event_hdl_sub_mgmt *mgmt)
 struct event_hdl_sub *event_hdl_subscribe_ptr(event_hdl_sub_list *sub_list,
                                               struct event_hdl_sub_type e_type, struct event_hdl hdl)
 {
-	struct event_hdl_sub *new_sub;
+	struct event_hdl_sub *new_sub = NULL;
 	struct mt_list *elt1, elt2;
-	uint8_t found = 0;
-	struct event_hdl_async_task_default_ctx	*task_ctx;
+	struct event_hdl_async_task_default_ctx	*task_ctx = NULL;
 
 	if (!sub_list)
 		sub_list = &global_event_hdl_sub_list; /* fall back to global list */
@@ -387,26 +392,9 @@ struct event_hdl_sub *event_hdl_subscribe_ptr(event_hdl_sub_list *sub_list,
 	       (hdl.async == EVENT_HDL_ASYNC_MODE_ADVANCED &&
 		(!hdl.async_equeue || !hdl.async_task)));
 
-	/* first check if such identified hdl is not already registered */
-	if (hdl.id) {
-		mt_list_for_each_entry_safe(new_sub, sub_list, mt_list, elt1, elt2) {
-			if (hdl.id == new_sub->hdl.id) {
-				/* we found matching registered hdl */
-				found = 1;
-				break;
-			}
-		}
-	}
-
-	if (found) {
-		/* error already registered */
-		event_hdl_report_hdl_state(ha_warning, &hdl, "SUB", "could not subscribe: subscription with this id already exists");
-		return NULL;
-	}
-
 	new_sub = pool_alloc(pool_head_sub);
 	if (new_sub == NULL) {
-		goto new_sub_memory_error;
+		goto memory_error;
 	}
 
 	/* assignments */
@@ -419,7 +407,7 @@ struct event_hdl_sub *event_hdl_subscribe_ptr(event_hdl_sub_list *sub_list,
 		new_sub->async_end = pool_alloc(pool_head_sub_event);
 		if (!new_sub->async_end) {
 			/* memory error */
-			goto new_sub_memory_error_event_end;
+			goto memory_error;
 		}
 		if (hdl.async == EVENT_HDL_ASYNC_MODE_NORMAL) {
 			/* normal mode: no task provided, we must initialize it */
@@ -429,7 +417,7 @@ struct event_hdl_sub *event_hdl_subscribe_ptr(event_hdl_sub_list *sub_list,
 
 			if (!task_ctx) {
 				/* memory error */
-				goto new_sub_memory_error_task_ctx;
+				goto memory_error;
 			}
 			MT_LIST_INIT(&task_ctx->e_queue);
 			task_ctx->func = new_sub->hdl.async_ptr;
@@ -439,13 +427,11 @@ struct event_hdl_sub *event_hdl_subscribe_ptr(event_hdl_sub_list *sub_list,
 
 			if (!new_sub->hdl.async_task) {
 				/* memory error */
-				goto new_sub_memory_error_task;
+				goto memory_error;
 			}
 			new_sub->hdl.async_task->context = task_ctx;
 			new_sub->hdl.async_task->process = event_hdl_async_task_default;
 		}
-		/* registration cannot fail anymore */
-
 		/* initialize END event (used to notify about subscription ending)
 		 * used by both normal and advanced mode:
 		 * 	- to safely terminate the task in normal mode
@@ -464,32 +450,65 @@ struct event_hdl_sub *event_hdl_subscribe_ptr(event_hdl_sub_list *sub_list,
 	 */
 	new_sub->refcount = 2;
 
+	/* ready for registration */
+	MT_LIST_INIT(&new_sub->mt_list);
+
+	/* check if such identified hdl is not already registered */
+	if (hdl.id) {
+		struct event_hdl_sub *cur_sub;
+		uint8_t found = 0;
+
+		HA_SPIN_LOCK(EHDL_LOCK, &sub_list->insert_lock);
+		mt_list_for_each_entry_safe(cur_sub, &sub_list->head, mt_list, elt1, elt2) {
+			if (hdl.id == cur_sub->hdl.id) {
+				/* we found matching registered hdl */
+				found = 1;
+				break;
+			}
+		}
+		if (found) {
+			/* error already registered */
+			HA_SPIN_UNLOCK(EHDL_LOCK, &sub_list->insert_lock);
+			event_hdl_report_hdl_state(ha_alert, &hdl, "SUB", "could not subscribe: subscription with this id already exists");
+			goto cleanup;
+		}
+	}
+
 	/* Append in list (global or user specified list).
 	 * For now, append when sync mode, and insert when async mode
 	 * so that async handlers are executed first
 	 */
-	MT_LIST_INIT(&new_sub->mt_list);
 	if (hdl.async) {
 		/* async mode, insert at the beginning of the list */
-		MT_LIST_INSERT(sub_list, &new_sub->mt_list);
+		MT_LIST_INSERT(&sub_list->head, &new_sub->mt_list);
 	} else {
 		/* sync mode, append at the end of the list */
-		MT_LIST_APPEND(sub_list, &new_sub->mt_list);
+		MT_LIST_APPEND(&sub_list->head, &new_sub->mt_list);
 	}
 
+	if (hdl.id)
+		HA_SPIN_UNLOCK(EHDL_LOCK, &sub_list->insert_lock);
+
 	return new_sub;
 
-new_sub_memory_error_task:
-	pool_free(pool_head_sub_taskctx, task_ctx);
-new_sub_memory_error_task_ctx:
-	pool_free(pool_head_sub_event, new_sub->async_end);
-new_sub_memory_error_event_end:
-	pool_free(pool_head_sub, new_sub);
-new_sub_memory_error:
-
-	event_hdl_report_hdl_state(ha_warning, &hdl, "SUB", "could not register subscription due to memory error");
+ cleanup:
+	if (new_sub) {
+		if (hdl.async == EVENT_HDL_ASYNC_MODE_NORMAL) {
+			if (new_sub->hdl.async_task)
+				tasklet_free(new_sub->hdl.async_task);
+			if (task_ctx)
+				pool_free(pool_head_sub_taskctx, task_ctx);
+		}
+		if (hdl.async)
+			pool_free(pool_head_sub_event, new_sub->async_end);
+		pool_free(pool_head_sub, new_sub);
+	}
 
 	return NULL;
+
+ memory_error:
+	event_hdl_report_hdl_state(ha_warning, &hdl, "SUB", "could not register subscription due to memory error");
+	goto cleanup;
 }
 
 void event_hdl_take(struct event_hdl_sub *sub)
@@ -547,7 +566,7 @@ int event_hdl_lookup_unsubscribe(event_hdl_sub_list *sub_list,
 	if (!sub_list)
 		sub_list = &global_event_hdl_sub_list; /* fall back to global list */
 
-	mt_list_for_each_entry_safe(del_sub, sub_list, mt_list, elt1, elt2) {
+	mt_list_for_each_entry_safe(del_sub, &sub_list->head, mt_list, elt1, elt2) {
 		if (lookup_id == del_sub->hdl.id) {
 			/* we found matching registered hdl */
 			MT_LIST_DELETE_SAFE(elt1);
@@ -569,7 +588,7 @@ int event_hdl_lookup_resubscribe(event_hdl_sub_list *sub_list,
 	if (!sub_list)
 		sub_list = &global_event_hdl_sub_list; /* fall back to global list */
 
-	mt_list_for_each_entry_safe(cur_sub, sub_list, mt_list, elt1, elt2) {
+	mt_list_for_each_entry_safe(cur_sub, &sub_list->head, mt_list, elt1, elt2) {
 		if (lookup_id == cur_sub->hdl.id) {
 			/* we found matching registered hdl */
 			status = _event_hdl_resub(cur_sub, type);
@@ -589,7 +608,7 @@ struct event_hdl_sub *event_hdl_lookup_take(event_hdl_sub_list *sub_list,
 	if (!sub_list)
 		sub_list = &global_event_hdl_sub_list; /* fall back to global list */
 
-	mt_list_for_each_entry_safe(cur_sub, sub_list, mt_list, elt1, elt2) {
+	mt_list_for_each_entry_safe(cur_sub, &sub_list->head, mt_list, elt1, elt2) {
 		if (lookup_id == cur_sub->hdl.id) {
 			/* we found matching registered hdl */
 			event_hdl_take(cur_sub);
@@ -612,7 +631,7 @@ static int _event_hdl_publish(event_hdl_sub_list *sub_list, struct event_hdl_sub
 	struct event_hdl_async_event_data *async_data = NULL; /* reuse async data for multiple async hdls */
 	int error = 0;
 
-	mt_list_for_each_entry_safe(cur_sub, sub_list, mt_list, elt1, elt2) {
+	mt_list_for_each_entry_safe(cur_sub, &sub_list->head, mt_list, elt1, elt2) {
 		/* notify each function that has subscribed to sub_family.type */
 		if ((cur_sub->sub.family == e_type.family) &&
 		    ((cur_sub->sub.subtype & e_type.subtype) == e_type.subtype)) {
@@ -751,10 +770,13 @@ void event_hdl_sub_list_destroy(event_hdl_sub_list *sub_list)
 
 	if (!sub_list)
 		sub_list = &global_event_hdl_sub_list; /* fall back to global list */
-	mt_list_for_each_entry_safe(cur_sub, sub_list, mt_list, elt1, elt2) {
+	mt_list_for_each_entry_safe(cur_sub, &sub_list->head, mt_list, elt1, elt2) {
 		/* remove cur elem from list */
 		MT_LIST_DELETE_SAFE(elt1);
 		/* then free it */
 		_event_hdl_unsubscribe(cur_sub);
 	}
+	HA_SPIN_DESTROY(&sub_list->insert_lock);
 }
+
+INITCALL0(STG_INIT, event_hdl_init);

src/thread.c

@@ -444,6 +444,7 @@ static const char *lock_label(enum lock_label label)
 	case IDLE_CONNS_LOCK:      return "IDLE_CONNS";
 	case QUIC_LOCK:            return "QUIC";
 	case OCSP_LOCK:            return "OCSP";
+	case EHDL_LOCK:            return "EVENT_HDL";
 	case OTHER_LOCK:           return "OTHER";
 	case DEBUG1_LOCK:          return "DEBUG1";
 	case DEBUG2_LOCK:          return "DEBUG2";