Add commentary

src/core/iomgr/pollset_posix.c

@@ -228,31 +228,44 @@ void grpc_pollset_work(grpc_exec_ctx *exec_ctx, grpc_pollset *pollset,
   int added_worker = 0;
   int locked = 1;
   int queued_work = 0;
+  int keep_polling = 0;
   /* this must happen before we (potentially) drop pollset->mu */
   worker->next = worker->prev = NULL;
   worker->reevaluate_polling_on_wakeup = 0;
   /* TODO(ctiller): pool these */
   grpc_wakeup_fd_init(&worker->wakeup_fd);
+  /* If there's work waiting for the pollset to be idle, and the
+     pollset is idle, then do that work */
   if (!grpc_pollset_has_workers(pollset) &&
       !grpc_closure_list_empty(pollset->idle_jobs)) {
     grpc_exec_ctx_enqueue_list(exec_ctx, &pollset->idle_jobs);
     goto done;
   }
+  /* Check alarms - these are a global resource so we just ping
+     each time through on every pollset.
+     May update deadline to ensure timely wakeups.
+     TODO(ctiller): can this work be localized? */
   if (grpc_alarm_check(exec_ctx, now, &deadline)) {
     gpr_mu_unlock(&pollset->mu);
     locked = 0;
     goto done;
   }
+  /* If we're shutting down then we don't execute any extended work */
   if (pollset->shutting_down) {
     goto done;
   }
-  if (pollset->in_flight_cbs) {
   /* Give do_promote priority so we don't starve it out */
+  if (pollset->in_flight_cbs) {
     gpr_mu_unlock(&pollset->mu);
     locked = 0;
     goto done;
   }
-  for (;;) {
+  /* Start polling, and keep doing so while we're being asked to
+     re-evaluate our pollers (this allows poll() based pollers to
+     ensure they don't miss wakeups) */
+  keep_polling = 1;
+  while (keep_polling) {
+    keep_polling = 0;
     if (!pollset->kicked_without_pollers) {
       if (!added_worker) {
         push_front_worker(pollset, worker);
@@ -268,21 +281,29 @@ void grpc_pollset_work(grpc_exec_ctx *exec_ctx, grpc_pollset *pollset,
     } else {
       pollset->kicked_without_pollers = 0;
     }
+  /* Finished execution - start cleaning up.
+     Note that we may arrive here from outside the enclosing while() loop.
+     In that case we won't loop though as we haven't added worker to the
+     worker list, which means nobody could ask us to re-evaluate polling). */
   done:
     if (!locked) {
       queued_work |= grpc_exec_ctx_flush(exec_ctx);
       gpr_mu_lock(&pollset->mu);
       locked = 1;
     }
+    /* If we're forced to re-evaluate polling (via grpc_pollset_kick with
+       GRPC_POLLSET_REEVALUATE_POLLING_ON_WAKEUP) then we land here and force
+       a loop */
     if (worker->reevaluate_polling_on_wakeup) {
       worker->reevaluate_polling_on_wakeup = 0;
       pollset->kicked_without_pollers = 0;
       if (queued_work) {
+        /* If there's queued work on the list, then set the deadline to be
+           immediate so we get back out of the polling loop quickly */
         deadline = gpr_inf_past(GPR_CLOCK_MONOTONIC);
       }
-      continue;
+      keep_polling = 1;
     }
-    break;
   }
   if (added_worker) {
     remove_worker(pollset, worker);