X-Git-Url: http://git.liburcu.org/?a=blobdiff_plain;f=lib%2Fringbuffer%2Fring_buffer_frontend.c;h=38dbb937462d3140365b6cb22599d9de24a083c6;hb=d78fe19d2fd65bf438cbcf9ef362c4803a71fedf;hp=92aa78f57ddf2ed95f42c16e23728d32f58291b6;hpb=d2bd780e91af600f4b0126b5ff4f560a44a4a32e;p=lttng-modules.git

diff --git a/lib/ringbuffer/ring_buffer_frontend.c b/lib/ringbuffer/ring_buffer_frontend.c
index 92aa78f5..38dbb937 100644
--- a/lib/ringbuffer/ring_buffer_frontend.c
+++ b/lib/ringbuffer/ring_buffer_frontend.c
@@ -1572,9 +1572,10 @@ 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;
@@ -1606,7 +1607,7 @@ 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.
@@ -1617,12 +1618,31 @@ 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 &&
@@ -1647,9 +1667,10 @@ 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;