Fix: add "flush empty" ioctl for stream intersection

Changing the behavior of the "snapshot" lttng command to implicitly do a
buffer "flush" (even when current packet is empty) had unwanted
side-effects: for instance, the snapshot ABI is used by the live timer
to grab the buffer positions, and we don't want to generate useless
empty packets in that scenario.

Therefore, add the "flush empty" behavior as a new ioctl to the ring
buffer. This allows lttng-tools to perform buffer flush (even for empty
packets) when it needs to. Given that this new ioctl is added within
stable branches as well, lttng-tools always need to handle "-ENOSYS"
gracefully.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

lib/ringbuffer/frontend_internal.h

@@ -167,6 +167,8 @@ void lib_ring_buffer_check_deliver_slow(const struct lib_ring_buffer_config *con
 
 extern
 void lib_ring_buffer_switch_remote(struct lib_ring_buffer *buf);
+extern
+void lib_ring_buffer_switch_remote_empty(struct lib_ring_buffer *buf);
 
 /* Buffer write helpers */
 

lib/ringbuffer/ring_buffer_frontend.c

@@ -1861,12 +1861,20 @@ static void _lib_ring_buffer_switch_remote(struct lib_ring_buffer *buf,
 	put_online_cpus();
 }
 
+/* Switch sub-buffer if current sub-buffer is non-empty. */
 void lib_ring_buffer_switch_remote(struct lib_ring_buffer *buf)
 {
 	_lib_ring_buffer_switch_remote(buf, SWITCH_ACTIVE);
 }
 EXPORT_SYMBOL_GPL(lib_ring_buffer_switch_remote);
 
+/* Switch sub-buffer even if current sub-buffer is empty. */
+void lib_ring_buffer_switch_remote_empty(struct lib_ring_buffer *buf)
+{
+	_lib_ring_buffer_switch_remote(buf, SWITCH_FLUSH);
+}
+EXPORT_SYMBOL_GPL(lib_ring_buffer_switch_remote_empty);
+
 /*
  * Returns :
  * 0 if ok

lib/ringbuffer/ring_buffer_vfs.c

@@ -273,6 +273,9 @@ long lib_ring_buffer_ioctl(struct file *filp, unsigned int cmd,
 	case RING_BUFFER_FLUSH:
 		lib_ring_buffer_switch_remote(buf);
 		return 0;
+	case RING_BUFFER_FLUSH_EMPTY:
+		lib_ring_buffer_switch_remote_empty(buf);
+		return 0;
 	default:
 		return -ENOIOCTLCMD;
 	}
@@ -421,6 +424,9 @@ long lib_ring_buffer_compat_ioctl(struct file *filp, unsigned int cmd,
 	case RING_BUFFER_COMPAT_FLUSH:
 		lib_ring_buffer_switch_remote(buf);
 		return 0;
+	case RING_BUFFER_COMPAT_FLUSH_EMPTY:
+		lib_ring_buffer_switch_remote_empty(buf);
+		return 0;
 	default:
 		return -ENOIOCTLCMD;
 	}

lib/ringbuffer/vfs.h

@@ -111,7 +111,7 @@ ssize_t vfs_lib_ring_buffer_splice_read(struct file *in, loff_t *ppos,
 #define RING_BUFFER_GET_MMAP_LEN		_IOR(0xF6, 0x0A, unsigned long)
 /* returns the offset of the subbuffer belonging to the mmap reader. */
 #define RING_BUFFER_GET_MMAP_READ_OFFSET	_IOR(0xF6, 0x0B, unsigned long)
-/* flush the current sub-buffer */
+/* Flush the current sub-buffer, if non-empty. */
 #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)
@@ -121,6 +121,8 @@ ssize_t vfs_lib_ring_buffer_splice_read(struct file *in, loff_t *ppos,
  * sub-buffer.
  */
 #define RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS	_IO(0xF6, 0x0E)
+/* Flush the current sub-buffer, even if empty. */
+#define RING_BUFFER_FLUSH_EMPTY			_IO(0xF6, 0x0F)
 
 #ifdef CONFIG_COMPAT
 /* Get a snapshot of the current ring buffer producer and consumer positions */
@@ -151,7 +153,7 @@ ssize_t vfs_lib_ring_buffer_splice_read(struct file *in, loff_t *ppos,
 #define RING_BUFFER_COMPAT_GET_MMAP_LEN		_IOR(0xF6, 0x0A, compat_ulong_t)
 /* returns the offset of the subbuffer belonging to the mmap reader. */
 #define RING_BUFFER_COMPAT_GET_MMAP_READ_OFFSET	_IOR(0xF6, 0x0B, compat_ulong_t)
-/* flush the current sub-buffer */
+/* Flush the current sub-buffer, if non-empty. */
 #define RING_BUFFER_COMPAT_FLUSH		RING_BUFFER_FLUSH
 /* Get the current version of the metadata cache (after a get_next). */
 #define RING_BUFFER_COMPAT_GET_METADATA_VERSION	RING_BUFFER_GET_METADATA_VERSION
@@ -162,6 +164,9 @@ ssize_t vfs_lib_ring_buffer_splice_read(struct file *in, loff_t *ppos,
  */
 #define RING_BUFFER_COMPAT_SNAPSHOT_SAMPLE_POSITIONS	\
 	RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS
+/* Flush the current sub-buffer, even if empty. */
+#define RING_BUFFER_COMPAT_FLUSH_EMPTY			\
+	RING_BUFFER_FLUSH_EMPTY
 #endif /* CONFIG_COMPAT */
 
 #endif /* _LIB_RING_BUFFER_VFS_H */

lttng-abi.c

@@ -684,6 +684,7 @@ long lttng_metadata_ring_buffer_ioctl(struct file *filp,
 		 */
 		return -ENOSYS;
 	}
+	case RING_BUFFER_FLUSH_EMPTY:	/* Fall-through. */
 	case RING_BUFFER_FLUSH:
 	{
 		struct lttng_metadata_stream *stream = filp->private_data;
@@ -760,6 +761,7 @@ long lttng_metadata_ring_buffer_compat_ioctl(struct file *filp,
 		 */
 		return -ENOSYS;
 	}
+	case RING_BUFFER_FLUSH_EMPTY:	/* Fall-through. */
 	case RING_BUFFER_FLUSH:
 	{
 		struct lttng_metadata_stream *stream = filp->private_data;