+++ /dev/null
-/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
-
-/***
- This file is part of systemd.
-
- Copyright 2014 David Herrmann <dh.herrmann@gmail.com>
-
- systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 of the License, or
- (at your option) any later version.
-
- systemd is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public License
- along with systemd; If not, see <http://www.gnu.org/licenses/>.
-***/
-
-#include <errno.h>
-#include <fcntl.h>
-#include <poll.h>
-#include <stdbool.h>
-#include <stdint.h>
-#include <stdlib.h>
-#include <sys/eventfd.h>
-#include <sys/types.h>
-#include <unistd.h>
-
-#include "barrier.h"
-#include "macro.h"
-#include "util.h"
-
-/**
- * Barriers
- * This barrier implementation provides a simple synchronization method based
- * on file-descriptors that can safely be used between threads and processes. A
- * barrier object contains 2 shared counters based on eventfd. Both processes
- * can now place barriers and wait for the other end to reach a random or
- * specific barrier.
- * Barriers are numbered, so you can either wait for the other end to reach any
- * barrier or the last barrier that you placed. This way, you can use barriers
- * for one-way *and* full synchronization. Note that even-though barriers are
- * numbered, these numbers are internal and recycled once both sides reached the
- * same barrier (implemented as a simple signed counter). It is thus not
- * possible to address barriers by their ID.
- *
- * Barrier-API: Both ends can place as many barriers via barrier_place() as
- * they want and each pair of barriers on both sides will be implicitly linked.
- * Each side can use the barrier_wait/sync_*() family of calls to wait for the
- * other side to place a specific barrier. barrier_wait_next() waits until the
- * other side calls barrier_place(). No links between the barriers are
- * considered and this simply serves as most basic asynchronous barrier.
- * barrier_sync_next() is like barrier_wait_next() and waits for the other side
- * to place their next barrier via barrier_place(). However, it only waits for
- * barriers that are linked to a barrier we already placed. If the other side
- * already placed more barriers than we did, barrier_sync_next() returns
- * immediately.
- * barrier_sync() extends barrier_sync_next() and waits until the other end
- * placed as many barriers via barrier_place() as we did. If they already placed
- * as many as we did (or more), it returns immediately.
- *
- * Additionally to basic barriers, an abortion event is available.
- * barrier_abort() places an abortion event that cannot be undone. An abortion
- * immediately cancels all placed barriers and replaces them. Any running and
- * following wait/sync call besides barrier_wait_abortion() will immediately
- * return false on both sides (otherwise, they always return true).
- * barrier_abort() can be called multiple times on both ends and will be a
- * no-op if already called on this side.
- * barrier_wait_abortion() can be used to wait for the other side to call
- * barrier_abort() and is the only wait/sync call that does not return
- * immediately if we aborted outself. It only returns once the other side
- * called barrier_abort().
- *
- * Barriers can be used for in-process and inter-process synchronization.
- * However, for in-process synchronization you could just use mutexes.
- * Therefore, main target is IPC and we require both sides to *not* share the FD
- * table. If that's given, barriers provide target tracking: If the remote side
- * exit()s, an abortion event is implicitly queued on the other side. This way,
- * a sync/wait call will be woken up if the remote side crashed or exited
- * unexpectedly. However, note that these abortion events are only queued if the
- * barrier-queue has been drained. Therefore, it is safe to place a barrier and
- * exit. The other side can safely wait on the barrier even though the exit
- * queued an abortion event. Usually, the abortion event would overwrite the
- * barrier, however, that's not true for exit-abortion events. Those are only
- * queued if the barrier-queue is drained (thus, the receiving side has placed
- * more barriers than the remote side).
- */
-
-/**
- * barrier_create() - Initialize a barrier object
- * @obj: barrier to initialize
- *
- * This initializes a barrier object. The caller is responsible of allocating
- * the memory and keeping it valid. The memory does not have to be zeroed
- * beforehand.
- * Two eventfd objects are allocated for each barrier. If allocation fails, an
- * error is returned.
- *
- * If this function fails, the barrier is reset to an invalid state so it is
- * safe to call barrier_destroy() on the object regardless whether the
- * initialization succeeded or not.
- *
- * The caller is responsible to destroy the object via barrier_destroy() before
- * releasing the underlying memory.
- *
- * Returns: 0 on success, negative error code on failure.
- */
-int barrier_create(Barrier *b) {
- _cleanup_(barrier_destroyp) Barrier *staging = b;
- int r;
-
- assert(b);
-
- b->me = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
- if (b->me < 0)
- return -errno;
-
- b->them = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
- if (b->them < 0)
- return -errno;
-
- r = pipe2(b->pipe, O_CLOEXEC | O_NONBLOCK);
- if (r < 0)
- return -errno;
-
- staging = NULL;
- return 0;
-}
-
-/**
- * barrier_destroy() - Destroy a barrier object
- * @b: barrier to destroy or NULL
- *
- * This destroys a barrier object that has previously been passed to
- * barrier_create(). The object is released and reset to invalid
- * state. Therefore, it is safe to call barrier_destroy() multiple
- * times or even if barrier_create() failed. However, barrier must be
- * always initialized with BARRIER_NULL.
- *
- * If @b is NULL, this is a no-op.
- */
-void barrier_destroy(Barrier *b) {
- if (!b)
- return;
-
- b->me = safe_close(b->me);
- b->them = safe_close(b->them);
- safe_close_pair(b->pipe);
- b->barriers = 0;
-}
-
-/**
- * barrier_set_role() - Set the local role of the barrier
- * @b: barrier to operate on
- * @role: role to set on the barrier
- *
- * This sets the roles on a barrier object. This is needed to know
- * which side of the barrier you're on. Usually, the parent creates
- * the barrier via barrier_create() and then calls fork() or clone().
- * Therefore, the FDs are duplicated and the child retains the same
- * barrier object.
- *
- * Both sides need to call barrier_set_role() after fork() or clone()
- * are done. If this is not done, barriers will not work correctly.
- *
- * Note that barriers could be supported without fork() or clone(). However,
- * this is currently not needed so it hasn't been implemented.
- */
-void barrier_set_role(Barrier *b, unsigned int role) {
- int fd;
-
- assert(b);
- assert(role == BARRIER_PARENT || role == BARRIER_CHILD);
- /* make sure this is only called once */
- assert(b->pipe[0] >= 0 && b->pipe[1] >= 0);
-
- if (role == BARRIER_PARENT)
- b->pipe[1] = safe_close(b->pipe[1]);
- else {
- b->pipe[0] = safe_close(b->pipe[0]);
-
- /* swap me/them for children */
- fd = b->me;
- b->me = b->them;
- b->them = fd;
- }
-}
-
-/* places barrier; returns false if we aborted, otherwise true */
-static bool barrier_write(Barrier *b, uint64_t buf) {
- ssize_t len;
-
- /* prevent new sync-points if we already aborted */
- if (barrier_i_aborted(b))
- return false;
-
- do {
- len = write(b->me, &buf, sizeof(buf));
- } while (len < 0 && IN_SET(errno, EAGAIN, EINTR));
-
- if (len != sizeof(buf))
- goto error;
-
- /* lock if we aborted */
- if (buf >= (uint64_t)BARRIER_ABORTION) {
- if (barrier_they_aborted(b))
- b->barriers = BARRIER_WE_ABORTED;
- else
- b->barriers = BARRIER_I_ABORTED;
- } else if (!barrier_is_aborted(b))
- b->barriers += buf;
-
- return !barrier_i_aborted(b);
-
-error:
- /* If there is an unexpected error, we have to make this fatal. There
- * is no way we can recover from sync-errors. Therefore, we close the
- * pipe-ends and treat this as abortion. The other end will notice the
- * pipe-close and treat it as abortion, too. */
-
- safe_close_pair(b->pipe);
- b->barriers = BARRIER_WE_ABORTED;
- return false;
-}
-
-/* waits for barriers; returns false if they aborted, otherwise true */
-static bool barrier_read(Barrier *b, int64_t comp) {
- if (barrier_they_aborted(b))
- return false;
-
- while (b->barriers > comp) {
- struct pollfd pfd[2] = {
- { .fd = b->pipe[0] >= 0 ? b->pipe[0] : b->pipe[1],
- .events = POLLHUP },
- { .fd = b->them,
- .events = POLLIN }};
- uint64_t buf;
- int r;
-
- r = poll(pfd, 2, -1);
- if (r < 0 && IN_SET(errno, EAGAIN, EINTR))
- continue;
- else if (r < 0)
- goto error;
-
- if (pfd[1].revents) {
- ssize_t len;
-
- /* events on @them signal new data for us */
- len = read(b->them, &buf, sizeof(buf));
- if (len < 0 && IN_SET(errno, EAGAIN, EINTR))
- continue;
-
- if (len != sizeof(buf))
- goto error;
- } else if (pfd[0].revents & (POLLHUP | POLLERR | POLLNVAL))
- /* POLLHUP on the pipe tells us the other side exited.
- * We treat this as implicit abortion. But we only
- * handle it if there's no event on the eventfd. This
- * guarantees that exit-abortions do not overwrite real
- * barriers. */
- buf = BARRIER_ABORTION;
- else
- continue;
-
- /* lock if they aborted */
- if (buf >= (uint64_t)BARRIER_ABORTION) {
- if (barrier_i_aborted(b))
- b->barriers = BARRIER_WE_ABORTED;
- else
- b->barriers = BARRIER_THEY_ABORTED;
- } else if (!barrier_is_aborted(b))
- b->barriers -= buf;
- }
-
- return !barrier_they_aborted(b);
-
-error:
- /* If there is an unexpected error, we have to make this fatal. There
- * is no way we can recover from sync-errors. Therefore, we close the
- * pipe-ends and treat this as abortion. The other end will notice the
- * pipe-close and treat it as abortion, too. */
-
- safe_close_pair(b->pipe);
- b->barriers = BARRIER_WE_ABORTED;
- return false;
-}
-
-/**
- * barrier_place() - Place a new barrier
- * @b: barrier object
- *
- * This places a new barrier on the barrier object. If either side already
- * aborted, this is a no-op and returns "false". Otherwise, the barrier is
- * placed and this returns "true".
- *
- * Returns: true if barrier was placed, false if either side aborted.
- */
-bool barrier_place(Barrier *b) {
- assert(b);
-
- if (barrier_is_aborted(b))
- return false;
-
- barrier_write(b, BARRIER_SINGLE);
- return true;
-}
-
-/**
- * barrier_abort() - Abort the synchronization
- * @b: barrier object to abort
- *
- * This aborts the barrier-synchronization. If barrier_abort() was already
- * called on this side, this is a no-op. Otherwise, the barrier is put into the
- * ABORT-state and will stay there. The other side is notified about the
- * abortion. Any following attempt to place normal barriers or to wait on normal
- * barriers will return immediately as "false".
- *
- * You can wait for the other side to call barrier_abort(), too. Use
- * barrier_wait_abortion() for that.
- *
- * Returns: false if the other side already aborted, true otherwise.
- */
-bool barrier_abort(Barrier *b) {
- assert(b);
-
- barrier_write(b, BARRIER_ABORTION);
- return !barrier_they_aborted(b);
-}
-
-/**
- * barrier_wait_next() - Wait for the next barrier of the other side
- * @b: barrier to operate on
- *
- * This waits until the other side places its next barrier. This is independent
- * of any barrier-links and just waits for any next barrier of the other side.
- *
- * If either side aborted, this returns false.
- *
- * Returns: false if either side aborted, true otherwise.
- */
-bool barrier_wait_next(Barrier *b) {
- assert(b);
-
- if (barrier_is_aborted(b))
- return false;
-
- barrier_read(b, b->barriers - 1);
- return !barrier_is_aborted(b);
-}
-
-/**
- * barrier_wait_abortion() - Wait for the other side to abort
- * @b: barrier to operate on
- *
- * This waits until the other side called barrier_abort(). This can be called
- * regardless whether the local side already called barrier_abort() or not.
- *
- * If the other side has already aborted, this returns immediately.
- *
- * Returns: false if the local side aborted, true otherwise.
- */
-bool barrier_wait_abortion(Barrier *b) {
- assert(b);
-
- barrier_read(b, BARRIER_THEY_ABORTED);
- return !barrier_i_aborted(b);
-}
-
-/**
- * barrier_sync_next() - Wait for the other side to place a next linked barrier
- * @b: barrier to operate on
- *
- * This is like barrier_wait_next() and waits for the other side to call
- * barrier_place(). However, this only waits for linked barriers. That means, if
- * the other side already placed more barriers than (or as much as) we did, this
- * returns immediately instead of waiting.
- *
- * If either side aborted, this returns false.
- *
- * Returns: false if either side aborted, true otherwise.
- */
-bool barrier_sync_next(Barrier *b) {
- assert(b);
-
- if (barrier_is_aborted(b))
- return false;
-
- barrier_read(b, MAX((int64_t)0, b->barriers - 1));
- return !barrier_is_aborted(b);
-}
-
-/**
- * barrier_sync() - Wait for the other side to place as many barriers as we did
- * @b: barrier to operate on
- *
- * This is like barrier_sync_next() but waits for the other side to call
- * barrier_place() as often as we did (in total). If they already placed as much
- * as we did (or more), this returns immediately instead of waiting.
- *
- * If either side aborted, this returns false.
- *
- * Returns: false if either side aborted, true otherwise.
- */
-bool barrier_sync(Barrier *b) {
- assert(b);
-
- if (barrier_is_aborted(b))
- return false;
-
- barrier_read(b, 0);
- return !barrier_is_aborted(b);
-}
~ianmdlvl / elogind.git / blobdiff