From a80a6b85b428e6ce12a8363bb1f08d44c50f3252 Mon Sep 17 00:00:00 2001 From: Andrew Morton Date: Thu, 8 Nov 2012 15:53:35 -0800 Subject: revert "epoll: support for disabling items, and a self-test app" Revert commit 03a7beb55b9f ("epoll: support for disabling items, and a self-test app") pending resolution of the issues identified by Michael Kerrisk, copied below. We'll revisit this for 3.8. : I've taken a look at this patch as it currently stands in 3.7-rc1, and : done a bit of testing. (By the way, the test program : tools/testing/selftests/epoll/test_epoll.c does not compile...) : : There are one or two places where the behavior seems a little strange, : so I have a question or two at the end of this mail. But other than : that, I want to check my understanding so that the interface can be : correctly documented. : : Just to go though my understanding, the problem is the following : scenario in a multithreaded application: : : 1. Multiple threads are performing epoll_wait() operations, : and maintaining a user-space cache that contains information : corresponding to each file descriptor being monitored by : epoll_wait(). : : 2. At some point, a thread wants to delete (EPOLL_CTL_DEL) : a file descriptor from the epoll interest list, and : delete the corresponding record from the user-space cache. : : 3. The problem with (2) is that some other thread may have : previously done an epoll_wait() that retrieved information : about the fd in question, and may be in the middle of using : information in the cache that relates to that fd. Thus, : there is a potential race. : : 4. The race can't solved purely in user space, because doing : so would require applying a mutex across the epoll_wait() : call, which would of course blow thread concurrency. : : Right? : : Your solution is the EPOLL_CTL_DISABLE operation. I want to : confirm my understanding about how to use this flag, since : the description that has accompanied the patches so far : has been a bit sparse : : 0. In the scenario you're concerned about, deleting a file : descriptor means (safely) doing the following: : (a) Deleting the file descriptor from the epoll interest list : using EPOLL_CTL_DEL : (b) Deleting the corresponding record in the user-space cache : : 1. It's only meaningful to use this EPOLL_CTL_DISABLE in : conjunction with EPOLLONESHOT. : : 2. Using EPOLL_CTL_DISABLE without using EPOLLONESHOT in : conjunction is a logical error. : : 3. The correct way to code multithreaded applications using : EPOLL_CTL_DISABLE and EPOLLONESHOT is as follows: : : a. All EPOLL_CTL_ADD and EPOLL_CTL_MOD operations should : should EPOLLONESHOT. : : b. When a thread wants to delete a file descriptor, it : should do the following: : : [1] Call epoll_ctl(EPOLL_CTL_DISABLE) : [2] If the return status from epoll_ctl(EPOLL_CTL_DISABLE) : was zero, then the file descriptor can be safely : deleted by the thread that made this call. : [3] If the epoll_ctl(EPOLL_CTL_DISABLE) fails with EBUSY, : then the descriptor is in use. In this case, the calling : thread should set a flag in the user-space cache to : indicate that the thread that is using the descriptor : should perform the deletion operation. : : Is all of the above correct? : : The implementation depends on checking on whether : (events & ~EP_PRIVATE_BITS) == 0 : This replies on the fact that EPOLL_CTL_AD and EPOLL_CTL_MOD always : set EPOLLHUP and EPOLLERR in the 'events' mask, and EPOLLONESHOT : causes those flags (as well as all others in ~EP_PRIVATE_BITS) to be : cleared. : : A corollary to the previous paragraph is that using EPOLL_CTL_DISABLE : is only useful in conjunction with EPOLLONESHOT. However, as things : stand, one can use EPOLL_CTL_DISABLE on a file descriptor that does : not have EPOLLONESHOT set in 'events' This results in the following : (slightly surprising) behavior: : : (a) The first call to epoll_ctl(EPOLL_CTL_DISABLE) returns 0 : (the indicator that the file descriptor can be safely deleted). : (b) The next call to epoll_ctl(EPOLL_CTL_DISABLE) fails with EBUSY. : : This doesn't seem particularly useful, and in fact is probably an : indication that the user made a logic error: they should only be using : epoll_ctl(EPOLL_CTL_DISABLE) on a file descriptor for which : EPOLLONESHOT was set in 'events'. If that is correct, then would it : not make sense to return an error to user space for this case? Cc: Michael Kerrisk Cc: "Paton J. Lewis" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- tools/testing/selftests/Makefile | 2 +- tools/testing/selftests/epoll/Makefile | 11 - tools/testing/selftests/epoll/test_epoll.c | 344 ----------------------------- 3 files changed, 1 insertion(+), 356 deletions(-) delete mode 100644 tools/testing/selftests/epoll/Makefile delete mode 100644 tools/testing/selftests/epoll/test_epoll.c (limited to 'tools/testing/selftests') diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile index 43480149119e..85baf11e2acd 100644 --- a/tools/testing/selftests/Makefile +++ b/tools/testing/selftests/Makefile @@ -1,4 +1,4 @@ -TARGETS = breakpoints kcmp mqueue vm cpu-hotplug memory-hotplug epoll +TARGETS = breakpoints kcmp mqueue vm cpu-hotplug memory-hotplug all: for TARGET in $(TARGETS); do \ diff --git a/tools/testing/selftests/epoll/Makefile b/tools/testing/selftests/epoll/Makefile deleted file mode 100644 index 19806ed62f50..000000000000 --- a/tools/testing/selftests/epoll/Makefile +++ /dev/null @@ -1,11 +0,0 @@ -# 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 deleted file mode 100644 index f7525392ce84..000000000000 --- a/tools/testing/selftests/epoll/test_epoll.c +++ /dev/null @@ -1,344 +0,0 @@ -/* - * 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 - * - */ - -#include -#include -#include -#include -#include -#include -#include -#include - -/* - * 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 (!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)) { - 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; -} -- cgit v1.2.3-54-g00ecf