Add ustctl_snapshot_sample_positions ustctl command

[lttng-ust.git] / libringbuffer / ring_buffer_frontend.c

diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c
index 6dd81e14b0bbb6e767b44687d725c210b43661fc..9b16685251ee1e1d8da49801c1f11ca61ec2060a 100644 (file)
--- 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