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 <mathieu.desnoyers@efficios.com>

diff --git a/lttng-ring-buffer-client.h b/lttng-ring-buffer-client.h
index dc3356e4d286f2c2d190d9816d4ab34f2eb65b84..d5c512c5ccb8c372845541296fa8bafd48fb46b2 100644 (file)
--- 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;
 }