X-Git-Url: http://git.lttng.org/?a=blobdiff_plain;f=libringbuffer%2Fring_buffer_frontend.c;h=bda0e1f497fecc786883726b6329ba51498240c9;hb=b07cd987c6856becc4ce1c20e5118b006f4431b0;hp=908539a8663973fde0a8e51384907b4690b0e30d;hpb=77d7fa989f79f3c2cb14d26576cbbb54b0637594;p=lttng-ust.git diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c index 908539a8..bda0e1f4 100644 --- a/libringbuffer/ring_buffer_frontend.c +++ b/libringbuffer/ring_buffer_frontend.c @@ -421,6 +421,113 @@ void lib_ring_buffer_channel_switch_timer(int sig, siginfo_t *si, void *uc) return; } +static +int lib_ring_buffer_poll_deliver(const struct lttng_ust_lib_ring_buffer_config *config, + struct lttng_ust_lib_ring_buffer *buf, + struct channel *chan, + struct lttng_ust_shm_handle *handle) +{ + unsigned long consumed_old, consumed_idx, commit_count, write_offset; + + consumed_old = uatomic_read(&buf->consumed); + consumed_idx = subbuf_index(consumed_old, chan); + commit_count = v_read(config, &shmp_index(handle, buf->commit_cold, consumed_idx)->cc_sb); + /* + * No memory barrier here, since we are only interested + * in a statistically correct polling result. The next poll will + * get the data is we are racing. The mb() that ensures correct + * memory order is in get_subbuf. + */ + write_offset = v_read(config, &buf->offset); + + /* + * Check that the subbuffer we are trying to consume has been + * already fully committed. + */ + + if (((commit_count - chan->backend.subbuf_size) + & chan->commit_count_mask) + - (buf_trunc(consumed_old, chan) + >> chan->backend.num_subbuf_order) + != 0) + return 0; + + /* + * Check that we are not about to read the same subbuffer in + * which the writer head is. + */ + if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed_old, chan) + == 0) + return 0; + + return 1; +} + +static +void lib_ring_buffer_wakeup(struct lttng_ust_lib_ring_buffer *buf, + struct lttng_ust_shm_handle *handle) +{ + int wakeup_fd = shm_get_wakeup_fd(handle, &buf->self._ref); + sigset_t sigpipe_set, pending_set, old_set; + int ret, sigpipe_was_pending = 0; + + if (wakeup_fd < 0) + return; + + /* + * 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); + } +} + static void lib_ring_buffer_channel_do_read(struct channel *chan) { @@ -2049,6 +2156,130 @@ int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx) return 0; } +static +void lib_ring_buffer_vmcore_check_deliver(const struct lttng_ust_lib_ring_buffer_config *config, + struct lttng_ust_lib_ring_buffer *buf, + unsigned long commit_count, + unsigned long idx, + struct lttng_ust_shm_handle *handle) +{ + if (config->oops == RING_BUFFER_OOPS_CONSISTENCY) + v_set(config, &shmp_index(handle, buf->commit_hot, idx)->seq, commit_count); +} + +void lib_ring_buffer_check_deliver_slow(const struct lttng_ust_lib_ring_buffer_config *config, + struct lttng_ust_lib_ring_buffer *buf, + struct channel *chan, + unsigned long offset, + unsigned long commit_count, + unsigned long idx, + struct lttng_ust_shm_handle *handle, + uint64_t tsc) +{ + unsigned long old_commit_count = commit_count + - chan->backend.subbuf_size; + + /* + * 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. + */ + + /* + * Order prior updates to reserve count prior to the + * commit_cold cc_sb update. + */ + cmm_smp_wmb(); + 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. + */ + 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); + + /* + * Increment the packet counter while we have exclusive + * access. + */ + subbuffer_inc_packet_count(config, &buf->backend, idx, 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); + /* + * Order later updates to reserve count after + * the commit cold cc_sb update. + */ + cmm_smp_wmb(); + 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) + && lib_ring_buffer_poll_deliver(config, buf, chan, handle)) { + lib_ring_buffer_wakeup(buf, handle); + } + } +} + /* * Force a read (imply TLS fixup for dlopen) of TLS variables. */