finish_consuming_dead_subbuffer: fix data_size read race, reread new consumed count

[ust.git] / libust / buffers.c

diff --git a/libust/buffers.c b/libust/buffers.c
index 732e03dce7263b33e19958d763607b8ff09f306f..9dcec2a825d953ab51a07adfdaf69e244c51e4b9 100644 (file)
--- a/libust/buffers.c
+++ b/libust/buffers.c
@@ -146,7 +146,13 @@ static void ltt_buffer_begin(struct ust_buffer *buf,
        header->cycle_count_begin = tsc;
        header->data_size = 0xFFFFFFFF; /* for recognizing crashed buffers */
        header->sb_size = 0xFFFFFFFF; /* for recognizing crashed buffers */
-       /* FIXME: add memory barrier? */
+       /*
+        * No memory barrier needed to order data_data/sb_size vs commit count
+        * update, because commit count update contains a compiler barrier that
+        * ensures the order of the writes are OK from a program POV. It only
+        * matters for crash dump recovery which is not executed concurrently,
+        * so memory write order does not matter.
+        */
        ltt_write_trace_header(channel->trace, header);
 }
 
@@ -356,7 +362,6 @@ static notrace void ltt_buffer_end(struct ust_buffer *buf,
                                subbuf_idx * buf->chan->subbuf_size);
        u32 data_size = SUBBUF_OFFSET(offset - 1, buf->chan) + 1;
 
-       header->data_size = data_size;
        header->sb_size = PAGE_ALIGN(data_size);
        header->cycle_count_end = tsc;
        header->events_lost = uatomic_read(&buf->events_lost);
@@ -364,6 +369,13 @@ static notrace void ltt_buffer_end(struct ust_buffer *buf,
        if(unlikely(header->events_lost > 0)) {
                DBG("Some events (%d) were lost in %s_%d", header->events_lost, buf->chan->channel_name, buf->cpu);
        }
+       /*
+        * Makes sure data_size write happens after write of the rest of the
+        * buffer end data, because data_size is used to identify a completely
+        * written subbuffer in a crash dump.
+        */
+       cmm_barrier();
+       header->data_size = data_size;
 }
 
 /*