Add RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS command

[lttng-modules.git] / lib / ringbuffer / ring_buffer_frontend.c

diff --git a/lib/ringbuffer/ring_buffer_frontend.c b/lib/ringbuffer/ring_buffer_frontend.c
index 68b62382e9fc87c2a307887cc67e144dfe1528a8..a477832ef45a9d30914d1dff1473d1e119e4e1d4 100644 (file)
--- a/lib/ringbuffer/ring_buffer_frontend.c
+++ b/lib/ringbuffer/ring_buffer_frontend.c
@@ -1119,6 +1119,37 @@ nodata:
 }
 EXPORT_SYMBOL_GPL(lib_ring_buffer_snapshot);
 
+/**
+ * Performs the same function as lib_ring_buffer_snapshot(), but the positions
+ * are saved regardless of whether the consumed and produced positions are
+ * in the same subbuffer.
+ * @buf: ring buffer
+ * @consumed: consumed byte count indicating the last position read
+ * @produced: produced byte count indicating the last position written
+ *
+ * This function is meant to provide information on the exact producer and
+ * consumer positions without regard for the "snapshot" feature.
+ */
+int lib_ring_buffer_snapshot_sample_positions(struct lib_ring_buffer *buf,
+               unsigned long *consumed, unsigned long *produced)
+{
+       struct channel *chan = buf->backend.chan;
+       const struct lib_ring_buffer_config *config = &chan->backend.config;
+
+       smp_rmb();
+       *consumed = atomic_long_read(&buf->consumed);
+       /*
+        * No need to issue a memory barrier between consumed count read and
+        * write offset read, because consumed count can only change
+        * concurrently in overwrite mode, and we keep a sequence counter
+        * identifier derived from the write offset to check we are getting
+        * the same sub-buffer we are expecting (the sub-buffers are atomically
+        * "tagged" upon writes, tags are checked upon read).
+        */
+       *produced = v_read(config, &buf->offset);
+       return 0;
+}
+
 /**
  * lib_ring_buffer_put_snapshot - move consumed counter forward
  *