lttng-clear: stop tracing required

Require that tracing is stopped when buffers are cleared. Update
comments and warning checks to that effect.

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

diff --git a/libringbuffer/frontend_internal.h b/libringbuffer/frontend_internal.h
index 1a9c2d4b78e82e7645e1267430b41e6f0c3218c2..7233ed6d91e7529c6af2aaa509f1d7e6e06532fc 100644 (file)
--- a/libringbuffer/frontend_internal.h
+++ b/libringbuffer/frontend_internal.h
@@ -205,6 +205,15 @@ void lib_ring_buffer_reserve_push_reader(struct lttng_ust_lib_ring_buffer *buf,
                                              consumed_new) != consumed_old));
 }
 
+/*
+ * Move consumed position to the beginning of subbuffer in which the
+ * write offset is. Should only be used on ring buffers that are not
+ * actively being written into, because clear_reader does not take into
+ * account the commit counters when moving the consumed position, which
+ * can make concurrent trace producers or consumers observe consumed
+ * position further than the write offset, which breaks ring buffer
+ * algorithm guarantees.
+ */
 static inline
 void lib_ring_buffer_clear_reader(struct lttng_ust_lib_ring_buffer *buf,
                                  struct lttng_ust_shm_handle *handle)
@@ -221,12 +230,10 @@ void lib_ring_buffer_clear_reader(struct lttng_ust_lib_ring_buffer *buf,
        do {
                offset = v_read(config, &buf->offset);
                consumed_old = uatomic_read(&buf->consumed);
-               if (caa_unlikely(subbuf_trunc(offset, chan)
-                             - subbuf_trunc(consumed_old, chan)
-                            > 0))
-                       consumed_new = subbuf_trunc(offset, chan);
-               else
-                       return;
+               CHAN_WARN_ON(chan, (long) (subbuf_trunc(offset, chan)
+                               - subbuf_trunc(consumed_old, chan))
+                               < 0);
+               consumed_new = subbuf_trunc(offset, chan);
        } while (caa_unlikely(uatomic_cmpxchg(&buf->consumed, consumed_old,
                                              consumed_new) != consumed_old));
 }