[PATCH] EPOLL_CTL_SET operatin for epoll_ctl when using EPOLLONESHOT

[Carlo Marcelo Arenas Belon <carenas@sajinet.com.pe>]

Simplify userspace by eliminating the need to track whether a specific
filehandle was added to epoll already and only needs to be re-arm
after being internally disabled by EPOLLONESHOT.

EPOLL_CLT_SET can be used to add or modify a filehandle in epoll
using the kernel's internal tracking to apply the right equivalent
operation to EPOLL_CTL_ADD or EPOLL_CTL_MOD.

Signed-off-by: Carlo Marcelo Arenas Belon <carenas@sajinet.com.pe>
---
 fs/eventpoll.c            |    9 ++++++++-
 include/linux/eventpoll.h |    1 +
 2 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 085c5c0..afa6972 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -799,7 +799,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
 	 * If the event mask does not contain any poll(2) event, we consider the
 	 * descriptor to be disabled. This condition is likely the effect of the
 	 * EPOLLONESHOT bit that disables the descriptor when an event is received,
-	 * until the next EPOLL_CTL_MOD will be issued.
+	 * until the next EPOLL_CTL_MOD or EPOLL_CTL_SET will be issued.
 	 */
 	if (!(epi->event.events & ~EP_PRIVATE_BITS))
 		goto out_unlock;
@@ -1302,6 +1302,13 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
 		} else
 			error = -ENOENT;
 		break;
+	case EPOLL_CTL_SET:
+		epds.events |= POLLERR | POLLHUP;
+		if (epi)
+			error = ep_modify(ep, epi, &epds);
+		else
+			error = ep_insert(ep, &epds, tfile, fd);
+		break;
 	}
 	mutex_unlock(&ep->mtx);
 
diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
index f6856a5..59758c0 100644
--- a/include/linux/eventpoll.h
+++ b/include/linux/eventpoll.h
@@ -25,6 +25,7 @@
 #define EPOLL_CTL_ADD 1
 #define EPOLL_CTL_DEL 2
 #define EPOLL_CTL_MOD 3
+#define EPOLL_CTL_SET 4
 
 /* Set the One Shot behaviour for the target file descriptor */
 #define EPOLLONESHOT (1 << 30)
-- 
1.6.3.3