/*-*- 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); }