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Diffstat (limited to 'src/shared/barrier.c')
| -rw-r--r-- | src/shared/barrier.c | 394 |
1 files changed, 394 insertions, 0 deletions
diff --git a/src/shared/barrier.c b/src/shared/barrier.c new file mode 100644 index 0000000..bb5869d --- /dev/null +++ b/src/shared/barrier.c @@ -0,0 +1,394 @@ +/* SPDX-License-Identifier: LGPL-2.1+ */ + +#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 "fd-util.h" +#include "macro.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 role) { + assert(b); + assert(IN_SET(role, BARRIER_PARENT, 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 */ + SWAP_TWO(b->me, b->them); + } +} + +/* 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; + + assert(b->me >= 0); + 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); +} |