X-Git-Url: http://git.lttng.org/?p=lttng-ust.git;a=blobdiff_plain;f=libringbuffer%2Fring_buffer_frontend.c;h=9b16685251ee1e1d8da49801c1f11ca61ec2060a;hp=6dd81e14b0bbb6e767b44687d725c210b43661fc;hb=f45930b75b297a5f80fe14ef2550b196ca750b4a;hpb=d0cba1f1d68f4aa7d611d4ba1c4c57f736f3c846

diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c
index 6dd81e14..9b166852 100644
--- a/libringbuffer/ring_buffer_frontend.c
+++ b/libringbuffer/ring_buffer_frontend.c
@@ -1305,6 +1305,44 @@ nodata:
 		return -EAGAIN;
 }
 
+/**
+ * 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 lttng_ust_lib_ring_buffer *buf,
+			     unsigned long *consumed, unsigned long *produced,
+			     struct lttng_ust_shm_handle *handle)
+{
+	struct channel *chan;
+	const struct lttng_ust_lib_ring_buffer_config *config;
+	unsigned long consumed_cur, write_offset;
+
+	chan = shmp(handle, buf->backend.chan);
+	if (!chan)
+		return -EPERM;
+	config = &chan->backend.config;
+	cmm_smp_rmb();
+	*consumed = uatomic_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_move_consumer - move consumed counter forward
  * @buf: ring buffer