[PATCH v3 10/13] epoll: support polling from userspace for ep_modify()

[Roman Penyaev]

When epfd is polled from userspace and item is being modified:

1. Update user item with new pointer or poll flags.
2. Add event to user ring if needed.

Signed-off-by: Roman Penyaev <rpeny...@suse.de>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: linux-fsde...@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 630f473973da..1b7097e58fb2 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2060,6 +2060,8 @@ static int ep_insert(struct eventpoll *ep, const struct 
epoll_event *event,
 static int ep_modify(struct eventpoll *ep, struct epitem *epi,
                     const struct epoll_event *event)
 {
+       struct epoll_uitem *uitem;
+       __poll_t revents;
        int pwake = 0;
        poll_table pt;
 
@@ -2074,6 +2076,14 @@ static int ep_modify(struct eventpoll *ep, struct epitem 
*epi,
         */
        epi->event.events = event->events; /* need barrier below */
        epi->event.data = event->data; /* protected by mtx */
+
+       /* Update user item, barrier is below */
+       if (ep_polled_by_user(ep)) {
+               uitem = &ep->user_header->items[epi->bit];
+               uitem->events = event->events;
+               uitem->data = event->data;
+       }
+
        if (epi->event.events & EPOLLWAKEUP) {
                if (!ep_has_wakeup_source(epi))
                        ep_create_wakeup_source(epi);
@@ -2107,12 +2117,19 @@ static int ep_modify(struct eventpoll *ep, struct 
epitem *epi,
         * If the item is "hot" and it is not registered inside the ready
         * list, push it inside.
         */
-       if (ep_item_poll(epi, &pt, 1)) {
+       revents = ep_item_poll(epi, &pt, 1);
+       if (revents) {
+               bool added = false;
+
                write_lock_irq(&ep->lock);
-               if (!ep_is_linked(epi)) {
+               if (ep_polled_by_user(ep))
+                       added = ep_add_event_to_uring(epi, revents);
+               else if (!ep_is_linked(epi)) {
                        list_add_tail(&epi->rdllink, &ep->rdllist);
                        ep_pm_stay_awake(epi);
-
+                       added = true;
+               }
+               if (added) {
                        /* Notify waiting tasks that events are available */
                        if (waitqueue_active(&ep->wq))
                                wake_up(&ep->wq);
-- 
2.21.0