fs/epoll: remove unnecessary wakeups of nested epoll

Take the case where we have:

        t0
         | (ew)
        e0
         | (et)
        e1
         | (lt)
        s0

t0: thread 0
e0: epoll fd 0
e1: epoll fd 1
s0: socket fd 0
ew: epoll_wait
et: edge-trigger
lt: level-trigger

We remove unnecessary wakeups to prevent the nested epoll that working in edge-
triggered mode to waking up continuously.

Test code:
 #include <unistd.h>
 #include <sys/epoll.h>
 #include <sys/socket.h>

 int main(int argc, char *argv[])
 {
 	int sfd[2];
 	int efd[2];
 	struct epoll_event e;

 	if (socketpair(AF_UNIX, SOCK_STREAM, 0, sfd) < 0)
 		goto out;

 	efd[0] = epoll_create(1);
 	if (efd[0] < 0)
 		goto out;

 	efd[1] = epoll_create(1);
 	if (efd[1] < 0)
 		goto out;

 	e.events = EPOLLIN;
 	if (epoll_ctl(efd[1], EPOLL_CTL_ADD, sfd[0], &e) < 0)
 		goto out;

 	e.events = EPOLLIN | EPOLLET;
 	if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
 		goto out;

 	if (write(sfd[1], "w", 1) != 1)
 		goto out;

 	if (epoll_wait(efd[0], &e, 1, 0) != 1)
 		goto out;

 	if (epoll_wait(efd[0], &e, 1, 0) != 0)
 		goto out;

 	close(efd[0]);
 	close(efd[1]);
 	close(sfd[0]);
 	close(sfd[1]);

 	return 0;

 out:
 	return -1;
 }

More tests:
 https://github.com/heiher/epoll-wakeup

Link: http://lkml.kernel.org/r/20191009060516.3577-1-r@hev.ccSigned-off-by: hev <r@hev.cc>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Eric Wong <e@80x24.org>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -666,7 +666,6 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
 			      void *priv, int depth, bool ep_locked)
 {
 	__poll_t res;
-	int pwake = 0;
 	struct epitem *epi, *nepi;
 	LIST_HEAD(txlist);
 
@@ -733,26 +732,11 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
 	 */
 	list_splice(&txlist, &ep->rdllist);
 	__pm_relax(ep->ws);
-
-	if (!list_empty(&ep->rdllist)) {
-		/*
-		 * Wake up (if active) both the eventpoll wait list and
-		 * the ->poll() wait list (delayed after we release the lock).
-		 */
-		if (waitqueue_active(&ep->wq))
-			wake_up(&ep->wq);
-		if (waitqueue_active(&ep->poll_wait))
-			pwake++;
-	}
 	write_unlock_irq(&ep->lock);
 
 	if (!ep_locked)
 		mutex_unlock(&ep->mtx);
 
-	/* We have to call this outside the lock */
-	if (pwake)
-		ep_poll_safewake(&ep->poll_wait);
-
 	return res;
 }