Fix: SWITCH_FLUSH new sub-buffer checks

The SWITCH_FLUSH, when performed on a completely empty sub-buffer, was
missing some checks (imported from space reservation).

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

diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c
index 4f8cc37189397c499886302b56ce4107ac8640bc..2bce0d1d94c7b244ce94039df7e8729ce6c72045 100644 (file)
--- a/libringbuffer/ring_buffer_frontend.c
+++ b/libringbuffer/ring_buffer_frontend.c
@@ -1449,10 +1449,11 @@ int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
                                    struct lttng_ust_lib_ring_buffer *buf,
                                    struct channel *chan,
                                    struct switch_offsets *offsets,
-                                   uint64_t *tsc)
+                                   uint64_t *tsc,
+                                   struct lttng_ust_shm_handle *handle)
 {
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
-       unsigned long off;
+       unsigned long off, reserve_commit_diff;
 
        offsets->begin = v_read(config, &buf->offset);
        offsets->old = offsets->begin;
@@ -1477,36 +1478,69 @@ int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
         * timestamps) are visible to the reader. This is required for
         * quiescence guarantees for the fusion merge.
         */
-       if (mode == SWITCH_FLUSH || off > 0) {
-               if (caa_unlikely(off == 0)) {
-                       /*
-                        * A final flush that encounters an empty
-                        * sub-buffer cannot switch buffer if a
-                        * reader is located within this sub-buffer.
-                        * Anyway, the purpose of final flushing of a
-                        * sub-buffer at offset 0 is to handle the case
-                        * of entirely empty stream.
-                        */
-                       if (caa_unlikely(subbuf_trunc(offsets->begin, chan)
-                                        - subbuf_trunc((unsigned long)
-                                            uatomic_read(&buf->consumed), chan)
-                                       >= chan->backend.buf_size))
-                               return -1;
-                       /*
-                        * The client does not save any header information.
-                        * Don't switch empty subbuffer on finalize, because it
-                        * is invalid to deliver a completely empty subbuffer.
-                        */
-                       if (!config->cb.subbuffer_header_size())
+       if (mode != SWITCH_FLUSH && !off)
+               return -1;      /* we do not have to switch : buffer is empty */
+
+       if (caa_unlikely(off == 0)) {
+               unsigned long sb_index, commit_count;
+
+               /*
+                * We are performing a SWITCH_FLUSH. At this stage, there are no
+                * concurrent writes into the buffer.
+                *
+                * The client does not save any header information.  Don't
+                * switch empty subbuffer on finalize, because it is invalid to
+                * deliver a completely empty subbuffer.
+                */
+               if (!config->cb.subbuffer_header_size())
+                       return -1;
+
+               /* Test new buffer integrity */
+               sb_index = subbuf_index(offsets->begin, chan);
+               commit_count = v_read(config,
+                               &shmp_index(handle, buf->commit_cold,
+                                       sb_index)->cc_sb);
+               reserve_commit_diff =
+                 (buf_trunc(offsets->begin, chan)
+                  >> chan->backend.num_subbuf_order)
+                 - (commit_count & chan->commit_count_mask);
+               if (caa_likely(reserve_commit_diff == 0)) {
+                       /* Next subbuffer not being written to. */
+                       if (caa_unlikely(config->mode != RING_BUFFER_OVERWRITE &&
+                               subbuf_trunc(offsets->begin, chan)
+                                - subbuf_trunc((unsigned long)
+                                    uatomic_read(&buf->consumed), chan)
+                               >= chan->backend.buf_size)) {
+                               /*
+                                * We do not overwrite non consumed buffers
+                                * and we are full : don't switch.
+                                */
                                return -1;
+                       } else {
+                               /*
+                                * Next subbuffer not being written to, and we
+                                * are either in overwrite mode or the buffer is
+                                * not full. It's safe to write in this new
+                                * subbuffer.
+                                */
+                       }
+               } else {
                        /*
-                        * Need to write the subbuffer start header on finalize.
+                        * Next subbuffer reserve offset does not match the
+                        * commit offset. Don't perform switch in
+                        * producer-consumer and overwrite mode.  Caused by
+                        * either a writer OOPS or too many nested writes over a
+                        * reserve/commit pair.
                         */
-                       offsets->switch_old_start = 1;
+                       return -1;
                }
-               offsets->begin = subbuf_align(offsets->begin, chan);
-       } else
-               return -1;      /* we do not have to switch : buffer is empty */
+
+               /*
+                * Need to write the subbuffer start header on finalize.
+                */
+               offsets->switch_old_start = 1;
+       }
+       offsets->begin = subbuf_align(offsets->begin, chan);
        /* Note: old points to the next subbuf at offset 0 */
        offsets->end = offsets->begin;
        return 0;
@@ -1536,7 +1570,7 @@ void lib_ring_buffer_switch_slow(struct lttng_ust_lib_ring_buffer *buf, enum swi
         */
        do {
                if (lib_ring_buffer_try_switch_slow(mode, buf, chan, &offsets,
-                                                   &tsc))
+                                                   &tsc, handle))
                        return; /* Switch not needed */
        } while (v_cmpxchg(config, &buf->offset, offsets.old, offsets.end)
                 != offsets.old);