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/lib/ringbuffer/frontend_internal.h b/lib/ringbuffer/frontend_internal.h
index a1bfe558440c7d8f7f5af4d45395d6047fe334b7..048386146c1177ee525792b0a4849f71fe87ebd9 100644 (file)
--- a/lib/ringbuffer/frontend_internal.h
+++ b/lib/ringbuffer/frontend_internal.h
@@ -189,7 +189,12 @@ void lib_ring_buffer_reserve_push_reader(struct lib_ring_buffer *buf,
 
 /*
  * Move consumed position to the beginning of subbuffer in which the
- * write offset is.
+ * 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 lib_ring_buffer *buf,
@@ -201,12 +206,10 @@ void lib_ring_buffer_clear_reader(struct lib_ring_buffer *buf,
        do {
                offset = v_read(config, &buf->offset);
                consumed_old = atomic_long_read(&buf->consumed);
-               if (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 (unlikely(atomic_long_cmpxchg(&buf->consumed, consumed_old,
                                              consumed_new) != consumed_old));
 }