[lttng-dev] [RFC PATCH lttng-modules] Add RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS command

[Jérémie Galarneau]

There is no need to bump the LTTNG_MODULES_ABI_MINOR_VERSION
since the multiple wildcard feature introduced as part of the 2.10
release already bumps it from 2 to 3.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau at efficios.com>
---
 lib/ringbuffer/frontend.h             |  4 ++++
 lib/ringbuffer/ring_buffer_frontend.c | 31 +++++++++++++++++++++++++++++++
 lib/ringbuffer/ring_buffer_vfs.c      |  3 +++
 lib/ringbuffer/vfs.h                  |  6 ++++++
 4 files changed, 44 insertions(+)

diff --git a/lib/ringbuffer/frontend.h b/lib/ringbuffer/frontend.h
index 6ff1545..909abc2 100644
--- a/lib/ringbuffer/frontend.h
+++ b/lib/ringbuffer/frontend.h
@@ -106,6 +106,10 @@ extern void lib_ring_buffer_release_read(struct lib_ring_buffer *buf);
 extern int lib_ring_buffer_snapshot(struct lib_ring_buffer *buf,
 				    unsigned long *consumed,
 				    unsigned long *produced);
+extern int lib_ring_buffer_snapshot_sample_positions(
+				    struct lib_ring_buffer *buf,
+				    unsigned long *consumed,
+				    unsigned long *produced);
 extern void lib_ring_buffer_move_consumer(struct lib_ring_buffer *buf,
 					  unsigned long consumed_new);
 
diff --git a/lib/ringbuffer/ring_buffer_frontend.c b/lib/ringbuffer/ring_buffer_frontend.c
index 68b6238..a477832 100644
--- a/lib/ringbuffer/ring_buffer_frontend.c
+++ b/lib/ringbuffer/ring_buffer_frontend.c
@@ -1120,6 +1120,37 @@ 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
  *
  * Should only be called from consumer context.
diff --git a/lib/ringbuffer/ring_buffer_vfs.c b/lib/ringbuffer/ring_buffer_vfs.c
index 15da212..603dcfc 100644
--- a/lib/ringbuffer/ring_buffer_vfs.c
+++ b/lib/ringbuffer/ring_buffer_vfs.c
@@ -202,6 +202,9 @@ long lib_ring_buffer_ioctl(struct file *filp, unsigned int cmd,
 			lib_ring_buffer_switch_remote_empty(buf);
 		return lib_ring_buffer_snapshot(buf, &buf->cons_snapshot,
 					    &buf->prod_snapshot);
+	case RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS:
+		return lib_ring_buffer_snapshot_sample_positions(buf,
+				&buf->cons_snapshot, &buf->prod_snapshot);
 	case RING_BUFFER_SNAPSHOT_GET_CONSUMED:
 		return put_ulong(buf->cons_snapshot, arg);
 	case RING_BUFFER_SNAPSHOT_GET_PRODUCED:
diff --git a/lib/ringbuffer/vfs.h b/lib/ringbuffer/vfs.h
index 5444a4b..8a89caf 100644
--- a/lib/ringbuffer/vfs.h
+++ b/lib/ringbuffer/vfs.h
@@ -115,6 +115,12 @@ ssize_t vfs_lib_ring_buffer_splice_read(struct file *in, loff_t *ppos,
 #define RING_BUFFER_FLUSH			_IO(0xF6, 0x0C)
 /* Get the current version of the metadata cache (after a get_next). */
 #define RING_BUFFER_GET_METADATA_VERSION	_IOR(0xF6, 0x0D, uint64_t)
+/*
+ * Get a snapshot of the current ring buffer producer and consumer positions,
+ * regardless of whether or not the two positions are contained within the same
+ * sub-buffer.
+ */
+#define RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS	_IO(0xF6, 0x0E)
 
 #ifdef CONFIG_COMPAT
 /* Get a snapshot of the current ring buffer producer and consumer positions */
-- 
2.12.2