Avoid race between readers and writer that could cause reader to read

uninitialized or unmapped data when accessing _reverseMappingIndices.

2 files changed, 13 insertions, 2 deletions

diff --git a/searchlib/src/vespa/searchlib/attribute/reference_mappings.cpp b/searchlib/src/vespa/searchlib/attribute/reference_mappings.cpp
index 03acf6f2167..4edd9d45e60 100644
--- a/searchlib/src/vespa/searchlib/attribute/reference_mappings.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/reference_mappings.cpp
@@ -9,6 +9,7 @@ namespace search::attribute {
 
 ReferenceMappings::ReferenceMappings(GenerationHolder &genHolder)
     : _reverseMappingIndices(genHolder),
+      _referencedLidLimit(0),
       _reverseMapping(),
       _referencedLids(genHolder)
 {
@@ -38,7 +39,6 @@ ReferenceMappings::syncForwardMapping(const Reference &entry)
                                          { referencedLids[lid] = referencedLid; });
 }
 
-
 void
 ReferenceMappings::syncReverseMappingIndices(const Reference &entry)
 {
@@ -46,6 +46,10 @@ ReferenceMappings::syncReverseMappingIndices(const Reference &entry)
     if (referencedLid != 0u) {
         _reverseMappingIndices.ensure_size(referencedLid + 1);
         _reverseMappingIndices[referencedLid] = entry.revMapIdx();
+        if (referencedLid >= _referencedLidLimit) {
+            std::atomic_thread_fence(std::memory_order_release);
+            _referencedLidLimit = referencedLid + 1;
+        }
     }
 }
 
diff --git a/searchlib/src/vespa/searchlib/attribute/reference_mappings.h b/searchlib/src/vespa/searchlib/attribute/reference_mappings.h
index 97fdb77e78d..3190e1b5a83 100644
--- a/searchlib/src/vespa/searchlib/attribute/reference_mappings.h
+++ b/searchlib/src/vespa/searchlib/attribute/reference_mappings.h
@@ -4,6 +4,7 @@
 
 #include <vespa/searchlib/btree/btreestore.h>
 #include <vespa/searchlib/common/rcuvector.h>
+#include <atomic>
 
 namespace search::attribute {
 
@@ -27,6 +28,8 @@ class ReferenceMappings
     // Vector containing references to trees of lids referencing given
     // referenced lid.
     ReverseMappingIndices _reverseMappingIndices;
+    // limit for referenced lid when accessing _reverseMappingIndices
+    uint32_t              _referencedLidLimit;
     // Store of B-Trees, used to map from gid or referenced lid to
     // referencing lids.
     ReverseMapping _reverseMapping;
@@ -77,7 +80,11 @@ public:
 
     ReferencedLids getReferencedLids() const { return ReferencedLids(&_referencedLids[0], _referencedLids.size()); }
     uint32_t getReferencedLid(uint32_t doc) const { return _referencedLids[doc]; }
-    ReverseMappingRefs getReverseMappingRefs() const { return ReverseMappingRefs(&_reverseMappingIndices[0], _reverseMappingIndices.size()); }
+    ReverseMappingRefs getReverseMappingRefs() const {
+        uint32_t referencedLidLimit = _referencedLidLimit;
+        std::atomic_thread_fence(std::memory_order_acquire);
+        return ReverseMappingRefs(&_reverseMappingIndices[0], referencedLidLimit);
+    }
     const ReverseMapping &getReverseMapping() const { return _reverseMapping; }
 };