[PATCH v4 10/14] 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
---
 fs/eventpoll.c | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index edf7ba28bce0..9f0d48eb360e 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2239,6 +2239,7 @@ 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)
 {
+       __poll_t revents;
        int pwake = 0;
        poll_table pt;
 
@@ -2250,10 +2251,24 @@ static int ep_modify(struct eventpoll *ep, struct 
epitem *epi,
         * Set the new event interest mask before calling f_op->poll();
         * otherwise we might miss an event that happens between the
         * f_op->poll() call and the new event set registering.
+        *
+        * Use xchg() here because we can race with ep_clear_public_event_bits()
+        * for the case when events are polled from userspace.  Internally
+        * ep_clear_public_event_bits() uses cmpxchg(), thus on some archs
+        * we can't mix normal writes and cmpxchg().
         */
-       epi->event.events = event->events; /* need barrier below */
+       xchg(&epi->event.events, event->events);
        epi->event.data = event->data; /* protected by mtx */
-       if (epi->event.events & EPOLLWAKEUP) {
+
+       /* Update user item, barrier is below */
+       if (ep_polled_by_user(ep)) {
+               struct uepitem *uepi = uep_item_from_epi(epi);
+               struct epoll_uitem *uitem;
+
+               uitem = &ep->user_header->items[uepi->bit];
+               WRITE_ONCE(uitem->events, event->events);
+               WRITE_ONCE(uitem->data, event->data);
+       } else if (epi->event.events & EPOLLWAKEUP) {
                if (!ep_has_wakeup_source(epi))
                        ep_create_wakeup_source(epi);
        } else if (ep_has_wakeup_source(epi)) {
@@ -2286,12 +2301,19 @@ 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