MINOR: h2: Document traps to be avoided on multithread.

Document a few traps to avoid if we ever attempt to allow the upper layer
of the mux h2 to be run by multiple threads.

diff --git a/src/mux_h2.c b/src/mux_h2.c
index 062754e39b0c4815ca9c437d65f647a0d158ec3d..14b6ba8f495dd7df83fbe944acf417846c69767f 100644 (file)
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -1239,6 +1239,10 @@ static inline void h2s_close(struct h2s *h2s)
 }
 
 /* detaches an H2 stream from its H2C and releases it to the H2S pool. */
+/* h2s_destroy should only ever be called by the thread that owns the stream,
+ * that means that a tasklet should be used if we want to destroy the h2s
+ * from another thread
+ */
 static void h2s_destroy(struct h2s *h2s)
 {
        struct connection *conn = h2s->h2c->conn;
@@ -1261,9 +1265,14 @@ static void h2s_destroy(struct h2s *h2s)
         */
        LIST_DEL_INIT(&h2s->list);
        if (LIST_ADDED(&h2s->sending_list)) {
+               /* It should be OK to call tasklet_remove_from_tasklet_list()
+                * here, as only the thread responsible for the tasklet should
+                * be called here.
+                */
                tasklet_remove_from_tasklet_list(h2s->send_wait->tasklet);
                LIST_DEL_INIT(&h2s->sending_list);
        }
+       /* ditto, calling tasklet_free() here should be ok */
        tasklet_free(h2s->wait_event.tasklet);
        pool_free(pool_head_h2s, h2s);
 
@@ -3809,6 +3818,11 @@ static void h2_detach(struct conn_stream *cs)
        /* The stream is about to die, so no need to attempt to run its task */
        if (LIST_ADDED(&h2s->sending_list) &&
            h2s->send_wait != &h2s->wait_event) {
+               /*
+                * Calling tasklet_remove_from_tasklet_list() here is fine,
+                * as only the thread responsible for the tasklet should
+                * call h2_detach().
+                */
                tasklet_remove_from_tasklet_list(h2s->send_wait->tasklet);
                LIST_DEL_INIT(&h2s->sending_list);
                /*
@@ -5628,6 +5642,9 @@ static int h2_unsubscribe(struct conn_stream *cs, int event_type, void *param)
                /* We were about to send, make sure it does not happen */
                if (LIST_ADDED(&h2s->sending_list) &&
                    h2s->send_wait != &h2s->wait_event) {
+                       /* This call should be ok, as only the thread responsible
+                        * for the tasklet should call unsubscribe.
+                        */
                        tasklet_remove_from_tasklet_list(h2s->send_wait->tasklet);
                        LIST_DEL_INIT(&h2s->sending_list);
                }
@@ -5720,6 +5737,11 @@ 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);
+               /* XXX: This won't work when/if streams are running on different
+                * threads, so we will have to find another way to prevent
+                * tasklets to run when we can no longer send data. The best
+                * may be to only wake enough tasklets to fill the buffer.
+                */
                tasklet_remove_from_tasklet_list(h2s->send_wait->tasklet);
                h2s->send_wait->events |= SUB_RETRY_SEND;
        }