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

[lttng-modules.git] / lib / ringbuffer / frontend_internal.h

diff --git a/lib/ringbuffer/frontend_internal.h b/lib/ringbuffer/frontend_internal.h
index bc284333f64c505e6e3fca646248073a8927f306..dbebdeec86f3362c30d59642ef9ab6910ca94a28 100644 (file)
--- a/lib/ringbuffer/frontend_internal.h
+++ b/lib/ringbuffer/frontend_internal.h
@@ -331,6 +331,12 @@ void lib_ring_buffer_check_deliver(const struct lib_ring_buffer_config *config,
                 * The subbuffer size is least 2 bytes (minimum size: 1 page).
                 * This guarantees that old_commit_count + 1 != commit_count.
                 */
+
+               /*
+                * Order prior updates to reserve count prior to the
+                * commit_cold cc_sb update.
+                */
+               smp_wmb();
                if (likely(v_cmpxchg(config, &buf->commit_cold[idx].cc_sb,
                                         old_commit_count, old_commit_count + 1)
                           == old_commit_count)) {
@@ -373,6 +379,11 @@ void lib_ring_buffer_check_deliver(const struct lib_ring_buffer_config *config,
                        /* End of exclusive subbuffer access */
                        v_set(config, &buf->commit_cold[idx].cc_sb,
                              commit_count);
+                       /*
+                        * Order later updates to reserve count after
+                        * the commit_cold cc_sb update.
+                        */
+                       smp_wmb();
                        lib_ring_buffer_vmcore_check_deliver(config, buf,
                                                         commit_count, idx);