Fix: ring buffer: handle concurrent update in nested buffer wrap around check

[lttng-modules.git] / lib / ringbuffer / ring_buffer_frontend.c

diff --git a/lib/ringbuffer/ring_buffer_frontend.c b/lib/ringbuffer/ring_buffer_frontend.c
index 942e14d3b9023d9ce76f797a834812329542b634..28525a691ec677ae14e028f3aeb065f3580a2445 100644 (file)
--- 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;