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Subject[PATCH v5 10/14] epoll: support polling from userspace for ep_modify()
Date
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 <rpenyaev@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-fsdevel@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 df96569d3b5a..f94608ca9f7a 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2265,6 +2265,7 @@ 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;

@@ -2276,10 +2277,24 @@ 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 */
+ (void) 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)) {
@@ -2312,12 +2327,19 @@ 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
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