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>

5 files changed, 392 insertions, 4 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index cd96649bfe62..da72250ddc1c 100644
--- a/fs/eventpoll.c
+++ b/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);
 
diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
index f4bb378ccf6a..41085d0f3955 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_DISABLE 4
 
 /*
  * Request the handling of system wakeup events so as to prevent system suspends
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 85baf11e2acd..43480149119e 100644
--- a/tools/testing/selftests/Makefile
+++ b/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 \
diff --git a/tools/testing/selftests/epoll/Makefile b/tools/testing/selftests/epoll/Makefile
new file mode 100644
index 000000000000..19806ed62f50
--- /dev/null
+++ b/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
diff --git a/tools/testing/selftests/epoll/test_epoll.c b/tools/testing/selftests/epoll/test_epoll.c
new file mode 100644
index 000000000000..e0fcff1e8331
--- /dev/null
+++ b/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;
+}