- - (old_commit_count & chan->commit_count_mask) == 0)) {
- /*
- * If we succeeded at updating cc_sb below, we are the subbuffer
- * writer delivering the subbuffer. Deals with concurrent
- * updates of the "cc" value without adding a add_return atomic
- * operation to the fast path.
- *
- * We are doing the delivery in two steps:
- * - First, we cmpxchg() cc_sb to the new value
- * old_commit_count + 1. This ensures that we are the only
- * subbuffer user successfully filling the subbuffer, but we
- * do _not_ set the cc_sb value to "commit_count" yet.
- * Therefore, other writers that would wrap around the ring
- * buffer and try to start writing to our subbuffer would
- * have to drop records, because it would appear as
- * non-filled.
- * We therefore have exclusive access to the subbuffer control
- * structures. This mutual exclusion with other writers is
- * crucially important to perform record overruns count in
- * flight recorder mode locklessly.
- * - When we are ready to release the subbuffer (either for
- * reading or for overrun by other writers), we simply set the
- * cc_sb value to "commit_count" and perform delivery.
- *
- * The subbuffer size is least 2 bytes (minimum size: 1 page).
- * This guarantees that old_commit_count + 1 != commit_count.
- */
- if (caa_likely(v_cmpxchg(config, &shmp_index(handle, buf->commit_cold, idx)->cc_sb,
- old_commit_count, old_commit_count + 1)
- == old_commit_count)) {
- /*
- * Start of exclusive subbuffer access. We are
- * guaranteed to be the last writer in this subbuffer
- * and any other writer trying to access this subbuffer
- * in this state is required to drop records.
- */
- tsc = config->cb.ring_buffer_clock_read(chan);
- v_add(config,
- subbuffer_get_records_count(config,
- &buf->backend,
- idx, handle),
- &buf->records_count);
- v_add(config,
- subbuffer_count_records_overrun(config,
- &buf->backend,
- idx, handle),
- &buf->records_overrun);
- config->cb.buffer_end(buf, tsc, idx,
- lib_ring_buffer_get_data_size(config,
- buf,
- idx,
- handle),
- handle);
-
- /*
- * Set noref flag and offset for this subbuffer id.
- * Contains a memory barrier that ensures counter stores
- * are ordered before set noref and offset.
- */
- lib_ring_buffer_set_noref_offset(config, &buf->backend, idx,
- buf_trunc_val(offset, chan), handle);
-
- /*
- * Order set_noref and record counter updates before the
- * end of subbuffer exclusive access. Orders with
- * respect to writers coming into the subbuffer after
- * wrap around, and also order wrt concurrent readers.
- */
- cmm_smp_mb();
- /* End of exclusive subbuffer access */
- v_set(config, &shmp_index(handle, buf->commit_cold, idx)->cc_sb,
- commit_count);
- lib_ring_buffer_vmcore_check_deliver(config, buf,
- commit_count, idx, handle);
-
- /*
- * RING_BUFFER_WAKEUP_BY_WRITER wakeup is not lock-free.
- */
- if (config->wakeup == RING_BUFFER_WAKEUP_BY_WRITER
- && (uatomic_read(&buf->active_readers)
- || uatomic_read(&buf->active_shadow_readers))
- && lib_ring_buffer_poll_deliver(config, buf, chan, handle)) {
- int wakeup_fd = shm_get_wakeup_fd(handle, &buf->self._ref);
-
- if (wakeup_fd >= 0) {
- sigset_t sigpipe_set, pending_set, old_set;
- int ret, sigpipe_was_pending = 0;
-
- /*
- * Wake-up the other end by
- * writing a null byte in the
- * pipe (non-blocking).
- * Important note: Because
- * writing into the pipe is
- * non-blocking (and therefore
- * we allow dropping wakeup
- * data, as long as there is
- * wakeup data present in the
- * pipe buffer to wake up the
- * consumer), the consumer
- * should perform the following
- * sequence for waiting:
- * 1) empty the pipe (reads).
- * 2) check if there is data in
- * the buffer.
- * 3) wait on the pipe (poll).
- *
- * Discard the SIGPIPE from write(), not
- * disturbing any SIGPIPE that might be
- * already pending. If a bogus SIGPIPE
- * is sent to the entire process
- * concurrently by a malicious user, it
- * may be simply discarded.
- */
- ret = sigemptyset(&pending_set);
- assert(!ret);
- /*
- * sigpending returns the mask
- * of signals that are _both_
- * blocked for the thread _and_
- * pending for either the thread
- * or the entire process.
- */
- ret = sigpending(&pending_set);
- assert(!ret);
- sigpipe_was_pending = sigismember(&pending_set, SIGPIPE);
- /*
- * If sigpipe was pending, it
- * means it was already blocked,
- * so no need to block it.
- */
- if (!sigpipe_was_pending) {
- ret = sigemptyset(&sigpipe_set);
- assert(!ret);
- ret = sigaddset(&sigpipe_set, SIGPIPE);
- assert(!ret);
- ret = pthread_sigmask(SIG_BLOCK, &sigpipe_set, &old_set);
- assert(!ret);
- }
- do {
- ret = write(wakeup_fd, "", 1);
- } while (ret == -1L && errno == EINTR);
- if (ret == -1L && errno == EPIPE && !sigpipe_was_pending) {
- struct timespec timeout = { 0, 0 };
- do {
- ret = sigtimedwait(&sigpipe_set, NULL,
- &timeout);
- } while (ret == -1L && errno == EINTR);
- }
- if (!sigpipe_was_pending) {
- ret = pthread_sigmask(SIG_SETMASK, &old_set, NULL);
- assert(!ret);
- }
- }
- }
- }
- }