epoll: possible bug from wakeup_source activation

[Eric Wong]

Hi Arve, looking at commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
(epoll: Add a flag, EPOLLWAKEUP, to prevent suspend ...)

I think the reason for using ep->ws instead of epi->ws in the unlikely
ovflist case applies to the likely rdllist case, too.  Since epi->ws is
only protected by ep->mtx, it can also be deactivated while inside
ep_poll_callback.

So something like the following patch might be necessary
(shown here with extra context):

--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -968,39 +968,45 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
 	if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
 		if (epi->next == EP_UNACTIVE_PTR) {
 			epi->next = ep->ovflist;
 			ep->ovflist = epi;
 			if (epi->ws) {
 				/*
 				 * Activate ep->ws since epi->ws may get
 				 * deactivated at any time.
 				 */
 				__pm_stay_awake(ep->ws);
 			}
 
 		}
 		goto out_unlock;
 	}
 
 	/* If this file is already in the ready list we exit soon */
 	if (!ep_is_linked(&epi->rdllink)) {
 		list_add_tail(&epi->rdllink, &ep->rdllist);
-		__pm_stay_awake(epi->ws);
+		if (epi->ws) {
+			/*
+			 * Activate ep->ws since epi->ws may get
+			 * deactivated at any time.
+			 */
+			__pm_stay_awake(ep->ws);
+		}
 	}
-----------------------------------------------------------------------
However, I think my proposed patch will also cause breakage with the
way epi->ws is handled in ep_send_events_proc:

		/*
		 * Activate ep->ws before deactivating epi->ws to prevent
		 * triggering auto-suspend here (in case we reactive epi->ws
		 * below).
		 *
		 * This could be rearranged to delay the deactivation of epi->ws
		 * instead, but then epi->ws would temporarily be out of sync
		 * with ep_is_linked().
		 */
		if (epi->ws && epi->ws->active)
			__pm_stay_awake(ep->ws);
		__pm_relax(epi->ws);
		list_del_init(&epi->rdllink);

I'm not sure, but maybe only using ep->ws is the easiest way to go.
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