Make MinimumUsedBitsTracker thread safe for both reads and writes

1 files changed, 22 insertions, 13 deletions

diff --git a/storage/src/vespa/storage/bucketdb/minimumusedbitstracker.h b/storage/src/vespa/storage/bucketdb/minimumusedbitstracker.h
index 881f7ed33cb..895446a31fe 100644
--- a/storage/src/vespa/storage/bucketdb/minimumusedbitstracker.h
+++ b/storage/src/vespa/storage/bucketdb/minimumusedbitstracker.h
@@ -1,8 +1,8 @@
 // Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
 #pragma once
 
-#include <algorithm>
 #include <vespa/document/bucket/bucketid.h>
+#include <atomic>
 
 namespace storage {
 
@@ -10,33 +10,42 @@ namespace storage {
  * Utility class for keeping track of the lowest used bits count seen
  * across a set of buckets.
  *
- * Not threadsafe by itself.
+ * Thread safe for reads and writes.
  */
 class MinimumUsedBitsTracker
 {
-    uint32_t _minUsedBits;
+    std::atomic<uint32_t> _min_used_bits;
 public:
-    MinimumUsedBitsTracker()
-        : _minUsedBits(58)
+    constexpr MinimumUsedBitsTracker() noexcept
+        : _min_used_bits(58)
     {}
 
     /**
-     * Returns true if new bucket led to a decrease in the used bits count.
+     * Returns true iff new bucket led to a decrease in the used bits count.
      */
-    bool update(const document::BucketId& bucket) {
-        if (bucket.getUsedBits() < _minUsedBits) {
-            _minUsedBits = bucket.getUsedBits();
+    bool update(const document::BucketId& bucket) noexcept {
+        const uint32_t bucket_bits = bucket.getUsedBits();
+        uint32_t current_bits = _min_used_bits.load(std::memory_order_relaxed);
+        if (bucket_bits < current_bits) {
+            while (!_min_used_bits.compare_exchange_strong(current_bits, bucket_bits,
+                                                           std::memory_order_relaxed,
+                                                           std::memory_order_relaxed))
+            {
+                if (bucket_bits >= current_bits) {
+                    return false; // We've raced with another writer that had lower or equal bits to our own bucket.
+                }
+            }
             return true;
         }
         return false;
     }
 
-    uint32_t getMinUsedBits() const {
-        return _minUsedBits;
+    [[nodiscard]] uint32_t getMinUsedBits() const noexcept {
+        return _min_used_bits.load(std::memory_order_relaxed);
     }
 
-    void setMinUsedBits(uint32_t minUsedBits) {
-        _minUsedBits = minUsedBits;
+    void setMinUsedBits(uint32_t minUsedBits) noexcept {
+        _min_used_bits.store(minUsedBits, std::memory_order_relaxed);
     }
 };