Fix: handle concurrent flush vs get_next_subbuf on metadata cache

[lttng-modules.git] / lttng-events.c

diff --git a/lttng-events.c b/lttng-events.c
index 701cff8ca8ff8663f82c0148bfb4ac15448bf81c..1fecff8efc279a76de3c8344f23236d485f7e5b8 100644 (file)
--- a/lttng-events.c
+++ b/lttng-events.c
@@ -597,16 +597,20 @@ int lttng_metadata_output_channel(struct lttng_metadata_stream *stream,
 
        /*
         * Ensure we support mutiple get_next / put sequences followed
-        * by put_next.
+        * by put_next. The metadata stream lock internally protects
+        * reading the metadata cache. It can indeed be read
+        * concurrently by "get_next_subbuf" and "flush" operations on
+        * the buffer invoked by different processes.
         */
+       mutex_lock(&stream->lock);
        WARN_ON(stream->metadata_in < stream->metadata_out);
        if (stream->metadata_in != stream->metadata_out)
-               return 0;
+               goto end;
 
        len = stream->metadata_cache->metadata_written -
                stream->metadata_in;
        if (!len)
-               return 0;
+               goto end;
        reserve_len = min_t(size_t,
                        stream->transport->ops.packet_avail_size(chan),
                        len);
@@ -628,6 +632,7 @@ int lttng_metadata_output_channel(struct lttng_metadata_stream *stream,
        ret = reserve_len;
 
 end:
+       mutex_unlock(&stream->lock);
        return ret;
 }