X-Git-Url: http://git.liburcu.org/?a=blobdiff_plain;f=lib%2Fringbuffer%2Fring_buffer_frontend.c;h=22cc62fcc02e7de82122801378a6bc6c3d89d7d1;hb=0fdec6861bdb4095e7537667f10131577eb7b279;hp=77f9e8bb4f560591f025d5b2b30b07fa4e1a777d;hpb=f5ea58003633d154b84c5df14a7e1fa7fcccb15c;p=lttng-modules.git

diff --git a/lib/ringbuffer/ring_buffer_frontend.c b/lib/ringbuffer/ring_buffer_frontend.c
index 77f9e8bb..22cc62fc 100644
--- a/lib/ringbuffer/ring_buffer_frontend.c
+++ b/lib/ringbuffer/ring_buffer_frontend.c
@@ -1352,8 +1352,10 @@ void lib_ring_buffer_switch_new_start(struct lib_ring_buffer *buf,
 /*
  * lib_ring_buffer_switch_new_end: finish switching current subbuffer
  *
- * The only remaining threads could be the ones with pending commits. They will
- * have to do the deliver themselves.
+ * Calls subbuffer_set_data_size() to set the data size of the current
+ * sub-buffer. We do not need to perform check_deliver nor commit here,
+ * since this task will be done by the "commit" of the event for which
+ * we are currently doing the space reservation.
  */
 static
 void lib_ring_buffer_switch_new_end(struct lib_ring_buffer *buf,
@@ -1362,33 +1364,11 @@ void lib_ring_buffer_switch_new_end(struct lib_ring_buffer *buf,
 					    u64 tsc)
 {
 	const struct lib_ring_buffer_config *config = &chan->backend.config;
-	unsigned long endidx = subbuf_index(offsets->end - 1, chan);
-	unsigned long commit_count, padding_size, data_size;
+	unsigned long endidx, data_size;
 
+	endidx = subbuf_index(offsets->end - 1, chan);
 	data_size = subbuf_offset(offsets->end - 1, chan) + 1;
-	padding_size = chan->backend.subbuf_size - data_size;
 	subbuffer_set_data_size(config, &buf->backend, endidx, data_size);
-
-	/*
-	 * Order all writes to buffer before the commit count update that will
-	 * determine that the subbuffer is full.
-	 */
-	if (config->ipi == RING_BUFFER_IPI_BARRIER) {
-		/*
-		 * Must write slot data before incrementing commit count.  This
-		 * compiler barrier is upgraded into a smp_mb() by the IPI sent
-		 * by get_subbuf().
-		 */
-		barrier();
-	} else
-		smp_wmb();
-	v_add(config, padding_size, &buf->commit_hot[endidx].cc);
-	commit_count = v_read(config, &buf->commit_hot[endidx].cc);
-	lib_ring_buffer_check_deliver(config, buf, chan, offsets->end - 1,
-				  commit_count, endidx);
-	lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
-					     offsets->end, commit_count,
-					     padding_size);
 }
 
 /*
@@ -1579,9 +1559,10 @@ int lib_ring_buffer_try_reserve_slow(struct lib_ring_buffer *buf,
 				     struct lib_ring_buffer_ctx *ctx)
 {
 	const struct lib_ring_buffer_config *config = &chan->backend.config;
-	unsigned long reserve_commit_diff;
+	unsigned long reserve_commit_diff, offset_cmp;
 
-	offsets->begin = v_read(config, &buf->offset);
+retry:
+	offsets->begin = offset_cmp = v_read(config, &buf->offset);
 	offsets->old = offsets->begin;
 	offsets->switch_new_start = 0;
 	offsets->switch_new_end = 0;
@@ -1613,7 +1594,7 @@ int lib_ring_buffer_try_reserve_slow(struct lib_ring_buffer *buf,
 		}
 	}
 	if (unlikely(offsets->switch_new_start)) {
-		unsigned long sb_index;
+		unsigned long sb_index, commit_count;
 
 		/*
 		 * We are typically not filling the previous buffer completely.
@@ -1624,12 +1605,31 @@ int lib_ring_buffer_try_reserve_slow(struct lib_ring_buffer *buf,
 				 + config->cb.subbuffer_header_size();
 		/* Test new buffer integrity */
 		sb_index = subbuf_index(offsets->begin, chan);
+		/*
+		 * Read buf->offset before buf->commit_cold[sb_index].cc_sb.
+		 * lib_ring_buffer_check_deliver() has the matching
+		 * memory barriers required around commit_cold cc_sb
+		 * updates to ensure reserve and commit counter updates
+		 * are not seen reordered when updated by another CPU.
+		 */
+		smp_rmb();
+		commit_count = v_read(config,
+				&buf->commit_cold[sb_index].cc_sb);
+		/* Read buf->commit_cold[sb_index].cc_sb before buf->offset. */
+		smp_rmb();
+		if (unlikely(offset_cmp != v_read(config, &buf->offset))) {
+			/*
+			 * The reserve counter have been concurrently updated
+			 * while we read the commit counter. This means the
+			 * commit counter we read might not match buf->offset
+			 * due to concurrent update. We therefore need to retry.
+			 */
+			goto retry;
+		}
 		reserve_commit_diff =
 		  (buf_trunc(offsets->begin, chan)
 		   >> chan->backend.num_subbuf_order)
-		  - ((unsigned long) v_read(config,
-					    &buf->commit_cold[sb_index].cc_sb)
-		     & chan->commit_count_mask);
+		  - (commit_count & chan->commit_count_mask);
 		if (likely(reserve_commit_diff == 0)) {
 			/* Next subbuffer not being written to. */
 			if (unlikely(config->mode != RING_BUFFER_OVERWRITE &&
@@ -1654,9 +1654,10 @@ int lib_ring_buffer_try_reserve_slow(struct lib_ring_buffer *buf,
 		} else {
 			/*
 			 * Next subbuffer reserve offset does not match the
-			 * commit offset. Drop record in producer-consumer and
-			 * overwrite mode. Caused by either a writer OOPS or too
-			 * many nested writes over a reserve/commit pair.
+			 * commit offset, and this did not involve update to the
+			 * reserve counter. Drop record in producer-consumer and
+			 * overwrite mode.  Caused by either a writer OOPS or
+			 * too many nested writes over a reserve/commit pair.
 			 */
 			v_inc(config, &buf->records_lost_wrap);
 			return -EIO;