From 59a49244b2cd0e06b15457a570ea7c5b3f0a5a53 Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Wed, 10 Apr 2019 10:25:47 -0400 Subject: [PATCH] Fix: don't access packet header for stream_id and stream_instance_id getters The stream ID and stream instance ID are invariant for a stream, so there is no point reading them from the packet header currently owned by the consumer (between get/put subbuf). Actually, the consumer try to access the stream_id from the live timer when sending a live beacon without getting the reader subbuffer first. Doing so is racy against producers. In typical live scenarios (non-overwrite channels), the producers will always write the same stream id and stream instance id values at the same header offsets, which will "work", except for the initial state of an empty buffer: the value "0" will be returned (erroneously). For the less frequently used scenario of a live session with "overwrite" channels, this will trigger WARN_ON safety nets in libringbuffer. This safety net triggers a kernel OOPS report and disables tracing for that channel. In the case where a ring buffer does not have any data ready, it makes no sense to try to get a subbuffer for reading anyway, so the approach was broken. So return the stream id and stream instance id from the internal data structures rather than reading it from the ring buffer. Signed-off-by: Mathieu Desnoyers --- lttng-ring-buffer-client.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/lttng-ring-buffer-client.h b/lttng-ring-buffer-client.h index dc3356e4..d5c512c5 100644 --- a/lttng-ring-buffer-client.h +++ b/lttng-ring-buffer-client.h @@ -479,9 +479,10 @@ static int client_stream_id(const struct lib_ring_buffer_config *config, struct lib_ring_buffer *buf, uint64_t *stream_id) { - struct packet_header *header = client_packet_header(config, buf); - *stream_id = header->stream_id; + struct channel *chan = buf->backend.chan; + struct lttng_channel *lttng_chan = channel_get_private(chan); + *stream_id = lttng_chan->id; return 0; } @@ -510,8 +511,7 @@ int client_instance_id(const struct lib_ring_buffer_config *config, struct lib_ring_buffer *buf, uint64_t *id) { - struct packet_header *header = client_packet_header(config, buf); - *id = header->stream_instance_id; + *id = buf->backend.cpu; return 0; } -- 2.34.1