[PATCH v3 11/13] epoll: support polling from userspace for ep_poll()

Roman Penyaev
2019-05-16T08:58:46Z

Rule of thumb for epfd polled from userspace is simple: epfd has
events if ->head != ->tail, no traversing of each item is performed.

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

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 1b7097e58fb2..20c94587488f 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -440,6 +440,12 @@ static inline bool ep_polled_by_user(struct eventpoll *ep)
return !!ep->user_header;
}

+static inline bool ep_uring_events_available(struct eventpoll *ep)
+{
+ return ep_polled_by_user(ep) &&
+ ep->user_header->head != ep->user_header->tail;
+}
+
/**
* ep_events_available - Checks if ready events might be available.
*
@@ -451,7 +457,8 @@ static inline bool ep_polled_by_user(struct eventpoll *ep)
static inline int ep_events_available(struct eventpoll *ep)
{
return !list_empty_careful(&ep->rdllist) ||
- READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
+ READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR ||
+ ep_uring_events_available(ep);
}

#ifdef CONFIG_NET_RX_BUSY_POLL
@@ -1160,7 +1167,7 @@ static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
int depth)
{
- struct eventpoll *ep;
+ struct eventpoll *ep, *tep;
bool locked;

pt->_key = epi->event.events;
@@ -1169,6 +1176,26 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,

ep = epi->ffd.file->private_data;
poll_wait(epi->ffd.file, &ep->poll_wait, pt);
+
+ tep = epi->ffd.file->private_data;
+ if (ep_polled_by_user(tep)) {
+ /*
+ * The behaviour differs comparing to full scan of ready
+ * list for original epoll. If descriptor is pollable
+ * from userspace we don't do scan of all ready user items:
+ * firstly because we can't do reverse search of epi by
+ * uitem bit, secondly this is simply waste of time for
+ * edge triggered descriptors (user code should be prepared
+ * to deal with EAGAIN returned from read() or write() on
+ * inserted file descriptor) and thirdly once event is put
+ * into user index ring do not touch it from kernel, what
+ * we do is mark it as EPOLLREMOVED on ep_remove() and
+ * that's it.
+ */
+ return ep_uring_events_available(tep) ?
+ EPOLLIN | EPOLLRDNORM : 0;
+ }
+
locked = pt && (pt->_qproc == ep_ptable_queue_proc);

return ep_scan_ready_list(epi->ffd.file->private_data,
@@ -1211,6 +1238,12 @@ static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)
/* Insert inside our poll wait queue */
poll_wait(file, &ep->poll_wait, wait);

+ if (ep_polled_by_user(ep)) {
+ /* Please read detailed comments inside ep_item_poll() */
+ return ep_uring_events_available(ep) ?
+ EPOLLIN | EPOLLRDNORM : 0;
+ }
+
/*
* Proceed to find out if wanted events are really available inside
* the ready list.
@@ -2235,6 +2268,8 @@ static int ep_send_events(struct eventpoll *ep,
{
struct ep_send_events_data esed;

+ WARN_ON(ep_polled_by_user(ep));
+
esed.maxevents = maxevents;
esed.events = events;

@@ -2281,6 +2316,12 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,

lockdep_assert_irqs_enabled();

+ if (ep_polled_by_user(ep)) {
+ if (ep_uring_events_available(ep))
+ /* Firstly all events from ring have to be consumed */
+ return -ESTALE;
+ }
+
if (timeout > 0) {
struct timespec64 end_time = ep_set_mstimeout(timeout);

@@ -2369,14 +2410,21 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
__set_current_state(TASK_RUNNING);

send_events:
- /*
- * Try to transfer events to user space. In case we get 0 events and
- * there's still timeout left over, we go trying again in search of
- * more luck.
- */
- if (!res && eavail &&
- !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
- goto fetch_events;
+ if (!res && eavail) {
+ if (!ep_polled_by_user(ep)) {
+ /*
+ * Try to transfer events to user space. In case we get
+ * 0 events and there's still timeout left over, we go
+ * trying again in search of more luck.
+ */
+ res = ep_send_events(ep, events, maxevents);
+ if (!res && !timed_out)
+ goto fetch_events;
+ } else {
+ /* User has to deal with the ring himself */
+ res = -ESTALE;
+ }
+ }

if (waiter) {
spin_lock_irq(&ep->wq.lock);
--
2.21.0