From c257a340ede0104e902807f0f001799850343ae9 Mon Sep 17 00:00:00 2001
From: David Rientjes <rientjes@google.com>
Date: Mon, 10 Jul 2017 15:52:33 -0700
Subject: [PATCH] fs, epoll: short circuit fetching events if thread has been killed
Git-commit: c257a340ede0104e902807f0f001799850343ae9
Patch-mainline: v4.13-rc1
References: bsc#1052639

We've encountered zombies that are waiting for a thread to exit that are
looping in ep_poll() almost endlessly although there is a pending
SIGKILL as a result of a group exit.

This happens because we always find ep_events_available() and fetch more
events and never are able to check for signal_pending() that would break
from the loop and return -EINTR.

Special case fatal signals and break immediately to guarantee that we
loop to fetch more events and delay making a timely exit.

It would also be possible to simply move the check for signal_pending()
higher than checking for ep_events_available(), but there have been no
reports of delayed signal handling other than SIGKILL preventing zombies
from exiting that would be fixed by this.

It fixes an issue for us where we have witnessed zombies sticking around
for at least O(minutes), but considering the code has been like this
forever and nobody else has complained that I have found, I would simply
queue it up for 4.12.

Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1705031722350.76784@chino.kir.corp.google.com
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.cz>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

---
 fs/eventpoll.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index b1c8e23ddf65..a6d194831ed8 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1748,6 +1748,16 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 			 * to TASK_INTERRUPTIBLE before doing the checks.
 			 */
 			set_current_state(TASK_INTERRUPTIBLE);
+			/*
+			 * Always short-circuit for fatal signals to allow
+			 * threads to make a timely exit without the chance of
+			 * finding more events available and fetching
+			 * repeatedly.
+			 */
+			if (fatal_signal_pending(current)) {
+				res = -EINTR;
+				break;
+			}
 			if (ep_events_available(ep) || timed_out)
 				break;
 			if (signal_pending(current)) {
-- 
2.12.0