epoll: support for disabling items, and a self-test app

Enhanced epoll_ctl to support EPOLL_CTL_DISABLE, which disables an epoll
item.  If epoll_ctl doesn't return -EBUSY in this case, it is then safe to
delete the epoll item in a multi-threaded environment.  Also added a new
test_epoll self- test app to both demonstrate the need for this feature
and test it.

Signed-off-by: Paton J. Lewis <palewis@adobe.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jason Baron <jbaron@redhat.com>
Cc: Paul Holland <pholland@adobe.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -346,7 +346,7 @@ static inline struct epitem *ep_item_from_epqueue(poll_table *p)
 /* Tells if the epoll_ctl(2) operation needs an event copy from userspace */
 static inline int ep_op_has_event(int op)
 {
-	return op != EPOLL_CTL_DEL;
+	return op == EPOLL_CTL_ADD || op == EPOLL_CTL_MOD;
 }
 
 /* Initialize the poll safe wake up structure */
@@ -676,6 +676,34 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
 	return 0;
 }
 
+/*
+ * Disables a "struct epitem" in the eventpoll set. Returns -EBUSY if the item
+ * had no event flags set, indicating that another thread may be currently
+ * handling that item's events (in the case that EPOLLONESHOT was being
+ * used). Otherwise a zero result indicates that the item has been disabled
+ * from receiving events. A disabled item may be re-enabled via
+ * EPOLL_CTL_MOD. Must be called with "mtx" held.
+ */
+static int ep_disable(struct eventpoll *ep, struct epitem *epi)
+{
+	int result = 0;
+	unsigned long flags;
+
+	spin_lock_irqsave(&ep->lock, flags);
+	if (epi->event.events & ~EP_PRIVATE_BITS) {
+		if (ep_is_linked(&epi->rdllink))
+			list_del_init(&epi->rdllink);
+		/* Ensure ep_poll_callback will not add epi back onto ready
+		   list: */
+		epi->event.events &= EP_PRIVATE_BITS;
+		}
+	else
+		result = -EBUSY;
+	spin_unlock_irqrestore(&ep->lock, flags);
+
+	return result;
+}
+
 static void ep_free(struct eventpoll *ep)
 {
 	struct rb_node *rbp;
@@ -1020,8 +1048,6 @@ static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
 	rb_insert_color(&epi->rbn, &ep->rbr);
 }
 
-
-
 #define PATH_ARR_SIZE 5
 /*
  * These are the number paths of length 1 to 5, that we are allowing to emanate
@@ -1787,6 +1813,12 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
 		} else
 			error = -ENOENT;
 		break;
+	case EPOLL_CTL_DISABLE:
+		if (epi)
+			error = ep_disable(ep, epi);
+		else
+			error = -ENOENT;
+		break;
 	}
 	mutex_unlock(&ep->mtx);
 

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_DISABLE 4
 
 /*
  * Request the handling of system wakeup events so as to prevent system suspends

tools/testing/selftests/Makefile

@@ -1,4 +1,4 @@
-TARGETS = breakpoints kcmp mqueue vm cpu-hotplug memory-hotplug
+TARGETS = breakpoints kcmp mqueue vm cpu-hotplug memory-hotplug epoll
 
 all:
 	for TARGET in $(TARGETS); do \

tools/testing/selftests/epoll/Makefile

@@ -0,0 +1,11 @@
+# Makefile for epoll selftests
+
+all: test_epoll
+%: %.c
+	gcc -pthread -g -o $@ $^
+
+run_tests: all
+	./test_epoll
+
+clean:
+	$(RM) test_epoll

tools/testing/selftests/epoll/test_epoll.c

@@ -0,0 +1,344 @@
+/*
+ *  tools/testing/selftests/epoll/test_epoll.c
+ *
+ *  Copyright 2012 Adobe Systems Incorporated
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Paton J. Lewis <palewis@adobe.com>
+ *
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/epoll.h>
+#include <sys/socket.h>
+
+/*
+ * A pointer to an epoll_item_private structure will be stored in the epoll
+ * item's event structure so that we can get access to the epoll_item_private
+ * data after calling epoll_wait:
+ */
+struct epoll_item_private {
+	int index;  /* Position of this struct within the epoll_items array. */
+	int fd;
+	uint32_t events;
+	pthread_mutex_t mutex;  /* Guards the following variables... */
+	int stop;
+	int status;  /* Stores any error encountered while handling item. */
+	/* The following variable allows us to test whether we have encountered
+	   a problem while attempting to cancel and delete the associated
+	   event. When the test program exits, 'deleted' should be exactly
+	   one. If it is greater than one, then the failed test reflects a real
+	   world situation where we would have tried to access the epoll item's
+	   private data after deleting it: */
+	int deleted;
+};
+
+struct epoll_item_private *epoll_items;
+
+/*
+ * Delete the specified item from the epoll set. In a real-world secneario this
+ * is where we would free the associated data structure, but in this testing
+ * environment we retain the structure so that we can test for double-deletion:
+ */
+void delete_item(int index)
+{
+	__sync_fetch_and_add(&epoll_items[index].deleted, 1);
+}
+
+/*
+ * A pointer to a read_thread_data structure will be passed as the argument to
+ * each read thread:
+ */
+struct read_thread_data {
+	int stop;
+	int status;  /* Indicates any error encountered by the read thread. */
+	int epoll_set;
+};
+
+/*
+ * The function executed by the read threads:
+ */
+void *read_thread_function(void *function_data)
+{
+	struct read_thread_data *thread_data =
+		(struct read_thread_data *)function_data;
+	struct epoll_event event_data;
+	struct epoll_item_private *item_data;
+	char socket_data;
+
+	/* Handle events until we encounter an error or this thread's 'stop'
+	   condition is set: */
+	while (1) {
+		int result = epoll_wait(thread_data->epoll_set,
+					&event_data,
+					1,	/* Number of desired events */
+					1000);  /* Timeout in ms */
+		if (result < 0) {
+			/* Breakpoints signal all threads. Ignore that while
+			   debugging: */
+			if (errno == EINTR)
+				continue;
+			thread_data->status = errno;
+			return 0;
+		} else if (thread_data->stop)
+			return 0;
+		else if (result == 0)  /* Timeout */
+			continue;
+
+		/* We need the mutex here because checking for the stop
+		   condition and re-enabling the epoll item need to be done
+		   together as one atomic operation when EPOLL_CTL_DISABLE is
+		   available: */
+		item_data = (struct epoll_item_private *)event_data.data.ptr;
+		pthread_mutex_lock(&item_data->mutex);
+
+		/* Remove the item from the epoll set if we want to stop
+		   handling that event: */
+		if (item_data->stop)
+			delete_item(item_data->index);
+		else {
+			/* Clear the data that was written to the other end of
+			   our non-blocking socket: */
+			do {
+				if (read(item_data->fd, &socket_data, 1) < 1) {
+					if ((errno == EAGAIN) ||
+					    (errno == EWOULDBLOCK))
+						break;
+					else
+						goto error_unlock;
+				}
+			} while (item_data->events & EPOLLET);
+
+			/* The item was one-shot, so re-enable it: */
+			event_data.events = item_data->events;
+			if (epoll_ctl(thread_data->epoll_set,
+						  EPOLL_CTL_MOD,
+						  item_data->fd,
+						  &event_data) < 0)
+				goto error_unlock;
+		}
+
+		pthread_mutex_unlock(&item_data->mutex);
+	}
+
+error_unlock:
+	thread_data->status = item_data->status = errno;
+	pthread_mutex_unlock(&item_data->mutex);
+	return 0;
+}
+
+/*
+ * A pointer to a write_thread_data structure will be passed as the argument to
+ * the write thread:
+ */
+struct write_thread_data {
+	int stop;
+	int status;  /* Indicates any error encountered by the write thread. */
+	int n_fds;
+	int *fds;
+};
+
+/*
+ * The function executed by the write thread. It writes a single byte to each
+ * socket in turn until the stop condition for this thread is set. If writing to
+ * a socket would block (i.e. errno was EAGAIN), we leave that socket alone for
+ * the moment and just move on to the next socket in the list. We don't care
+ * about the order in which we deliver events to the epoll set. In fact we don't
+ * care about the data we're writing to the pipes at all; we just want to
+ * trigger epoll events:
+ */
+void *write_thread_function(void *function_data)
+{
+	const char data = 'X';
+	int index;
+	struct write_thread_data *thread_data =
+		(struct write_thread_data *)function_data;
+	while (!write_thread_data->stop)
+		for (index = 0;
+		     !thread_data->stop && (index < thread_data->n_fds);
+		     ++index)
+			if ((write(thread_data->fds[index], &data, 1) < 1) &&
+				(errno != EAGAIN) &&
+				(errno != EWOULDBLOCK)) {
+				write_thread_data->status = errno;
+				return;
+			}
+}
+
+/*
+ * Arguments are currently ignored:
+ */
+int main(int argc, char **argv)
+{
+	const int n_read_threads = 100;
+	const int n_epoll_items = 500;
+	int index;
+	int epoll_set = epoll_create1(0);
+	struct write_thread_data write_thread_data = {
+		0, 0, n_epoll_items, malloc(n_epoll_items * sizeof(int))
+	};
+	struct read_thread_data *read_thread_data =
+		malloc(n_read_threads * sizeof(struct read_thread_data));
+	pthread_t *read_threads = malloc(n_read_threads * sizeof(pthread_t));
+	pthread_t write_thread;
+
+	printf("-----------------\n");
+	printf("Runing test_epoll\n");
+	printf("-----------------\n");
+
+	epoll_items = malloc(n_epoll_items * sizeof(struct epoll_item_private));
+
+	if (epoll_set < 0 || epoll_items == 0 || write_thread_data.fds == 0 ||
+		read_thread_data == 0 || read_threads == 0)
+		goto error;
+
+	if (sysconf(_SC_NPROCESSORS_ONLN) < 2) {
+		printf("Error: please run this test on a multi-core system.\n");
+		goto error;
+	}
+
+	/* Create the socket pairs and epoll items: */
+	for (index = 0; index < n_epoll_items; ++index) {
+		int socket_pair[2];
+		struct epoll_event event_data;
+		if (socketpair(AF_UNIX,
+			       SOCK_STREAM | SOCK_NONBLOCK,
+			       0,
+			       socket_pair) < 0)
+			goto error;
+		write_thread_data.fds[index] = socket_pair[0];
+		epoll_items[index].index = index;
+		epoll_items[index].fd = socket_pair[1];
+		if (pthread_mutex_init(&epoll_items[index].mutex, NULL) != 0)
+			goto error;
+		/* We always use EPOLLONESHOT because this test is currently
+		   structured to demonstrate the need for EPOLL_CTL_DISABLE,
+		   which only produces useful information in the EPOLLONESHOT
+		   case (without EPOLLONESHOT, calling epoll_ctl with
+		   EPOLL_CTL_DISABLE will never return EBUSY). If support for
+		   testing events without EPOLLONESHOT is desired, it should
+		   probably be implemented in a separate unit test. */
+		epoll_items[index].events = EPOLLIN | EPOLLONESHOT;
+		if (index < n_epoll_items / 2)
+			epoll_items[index].events |= EPOLLET;
+		epoll_items[index].stop = 0;
+		epoll_items[index].status = 0;
+		epoll_items[index].deleted = 0;
+		event_data.events = epoll_items[index].events;
+		event_data.data.ptr = &epoll_items[index];
+		if (epoll_ctl(epoll_set,
+			      EPOLL_CTL_ADD,
+			      epoll_items[index].fd,
+			      &event_data) < 0)
+			goto error;
+	}
+
+	/* Create and start the read threads: */
+	for (index = 0; index < n_read_threads; ++index) {
+		read_thread_data[index].stop = 0;
+		read_thread_data[index].status = 0;
+		read_thread_data[index].epoll_set = epoll_set;
+		if (pthread_create(&read_threads[index],
+				   NULL,
+				   read_thread_function,
+				   &read_thread_data[index]) != 0)
+			goto error;
+	}
+
+	if (pthread_create(&write_thread,
+			   NULL,
+			   write_thread_function,
+			   &write_thread_data) != 0)
+		goto error;
+
+	/* Cancel all event pollers: */
+#ifdef EPOLL_CTL_DISABLE
+	for (index = 0; index < n_epoll_items; ++index) {
+		pthread_mutex_lock(&epoll_items[index].mutex);
+		++epoll_items[index].stop;
+		if (epoll_ctl(epoll_set,
+			      EPOLL_CTL_DISABLE,
+			      epoll_items[index].fd,
+			      NULL) == 0)
+			delete_item(index);
+		else if (errno != EBUSY) {
+			pthread_mutex_unlock(&epoll_items[index].mutex);
+			goto error;
+		}
+		/* EBUSY means events were being handled; allow the other thread
+		   to delete the item. */
+		pthread_mutex_unlock(&epoll_items[index].mutex);
+	}
+#else
+	for (index = 0; index < n_epoll_items; ++index) {
+		pthread_mutex_lock(&epoll_items[index].mutex);
+		++epoll_items[index].stop;
+		pthread_mutex_unlock(&epoll_items[index].mutex);
+		/* Wait in case a thread running read_thread_function is
+		   currently executing code between epoll_wait and
+		   pthread_mutex_lock with this item. Note that a longer delay
+		   would make double-deletion less likely (at the expense of
+		   performance), but there is no guarantee that any delay would
+		   ever be sufficient. Note also that we delete all event
+		   pollers at once for testing purposes, but in a real-world
+		   environment we are likely to want to be able to cancel event
+		   pollers at arbitrary times. Therefore we can't improve this
+		   situation by just splitting this loop into two loops
+		   (i.e. signal 'stop' for all items, sleep, and then delete all
+		   items). We also can't fix the problem via EPOLL_CTL_DEL
+		   because that command can't prevent the case where some other
+		   thread is executing read_thread_function within the region
+		   mentioned above: */
+		usleep(1);
+		pthread_mutex_lock(&epoll_items[index].mutex);
+		if (!epoll_items[index].deleted)
+			delete_item(index);
+		pthread_mutex_unlock(&epoll_items[index].mutex);
+	}
+#endif
+
+	/* Shut down the read threads: */
+	for (index = 0; index < n_read_threads; ++index)
+		__sync_fetch_and_add(&read_thread_data[index].stop, 1);
+	for (index = 0; index < n_read_threads; ++index) {
+		if (pthread_join(read_threads[index], NULL) != 0)
+			goto error;
+		if (read_thread_data[index].status)
+			goto error;
+	}
+
+	/* Shut down the write thread: */
+	__sync_fetch_and_add(&write_thread_data.stop, 1);
+	if ((pthread_join(write_thread, NULL) != 0) || write_thread_data.status)
+		goto error;
+
+	/* Check for final error conditions: */
+	for (index = 0; index < n_epoll_items; ++index) {
+		if (epoll_items[index].status != 0)
+			goto error;
+		if (pthread_mutex_destroy(&epoll_items[index].mutex) < 0)
+			goto error;
+	}
+	for (index = 0; index < n_epoll_items; ++index)
+		if (epoll_items[index].deleted != 1) {
+			printf("Error: item data deleted %1d times.\n",
+				   epoll_items[index].deleted);
+			goto error;
+		}
+
+	printf("[PASS]\n");
+	return 0;
+
+ error:
+	printf("[FAIL]\n");
+	return errno;
+}