[scudo] Use a tryLock in secondary release to OS

compiler-rt/lib/scudo/standalone/secondary.h

@@ -206,11 +206,13 @@ class MapAllocatorCache {
     computePercentage(SuccessfulRetrieves, CallsToRetrieve, &Integral,
                       &Fractional);
     const s32 Interval = atomic_load_relaxed(&ReleaseToOsIntervalMs);
-    Str->append(
-        "Stats: MapAllocatorCache: EntriesCount: %zu, "
-        "MaxEntriesCount: %u, MaxEntrySize: %zu, ReleaseToOsIntervalMs = %d\n",
-        LRUEntries.size(), atomic_load_relaxed(&MaxEntriesCount),
-        atomic_load_relaxed(&MaxEntrySize), Interval >= 0 ? Interval : -1);
+    Str->append("Stats: MapAllocatorCache: EntriesCount: %zu, "
+                "MaxEntriesCount: %u, MaxEntrySize: %zu, ReleaseToOsSkips: "
+                "%zu, ReleaseToOsIntervalMs = %d\n",
+                LRUEntries.size(), atomic_load_relaxed(&MaxEntriesCount),
+                atomic_load_relaxed(&MaxEntrySize),
+                atomic_load_relaxed(&ReleaseToOsSkips),
+                Interval >= 0 ? Interval : -1);
     Str->append("Stats: CacheRetrievalStats: SuccessRate: %u/%u "
                 "(%zu.%02zu%%)\n",
                 SuccessfulRetrieves, CallsToRetrieve, Integral, Fractional);
@@ -343,8 +345,15 @@ class MapAllocatorCache {
       unmapCallBack(EvictMemMap);
 
     if (Interval >= 0) {
-      // TODO: Add ReleaseToOS logic to LRU algorithm
-      releaseOlderThan(Time - static_cast<u64>(Interval) * 1000000);
+      // It is very likely that multiple threads trying to do a release at the
+      // same time will not actually release any extra elements. Therefore,
+      // let any other thread continue, skipping the release.
+      if (Mutex.tryLock()) {
+        // TODO: Add ReleaseToOS logic to LRU algorithm
+        releaseOlderThan(Time - static_cast<u64>(Interval) * 1000000);
+        Mutex.unlock();
+      } else
+        atomic_fetch_add(&ReleaseToOsSkips, 1U, memory_order_relaxed);
     }
   }
 
@@ -488,7 +497,12 @@ class MapAllocatorCache {
     return true;
   }
 
-  void releaseToOS() { releaseOlderThan(UINT64_MAX); }
+  void releaseToOS() EXCLUDES(Mutex) {
+    // Since this is a request to release everything, always wait for the
+    // lock so that we guarantee all entries are released after this call.
+    ScopedLock L(Mutex);
+    releaseOlderThan(UINT64_MAX);
+  }
 
   void disableMemoryTagging() EXCLUDES(Mutex) {
     ScopedLock L(Mutex);
@@ -554,8 +568,7 @@ class MapAllocatorCache {
     Entry.Time = 0;
   }
 
-  void releaseOlderThan(u64 Time) EXCLUDES(Mutex) {
-    ScopedLock L(Mutex);
+  void releaseOlderThan(u64 Time) REQUIRES(Mutex) {
     if (!LRUEntries.size() || OldestTime == 0 || OldestTime > Time)
       return;
     OldestTime = 0;
@@ -573,6 +586,7 @@ class MapAllocatorCache {
   atomic_s32 ReleaseToOsIntervalMs = {};
   u32 CallsToRetrieve GUARDED_BY(Mutex) = 0;
   u32 SuccessfulRetrieves GUARDED_BY(Mutex) = 0;
+  atomic_uptr ReleaseToOsSkips = {};
 
   CachedBlock Entries[Config::getEntriesArraySize()] GUARDED_BY(Mutex) = {};
   NonZeroLengthArray<CachedBlock, Config::getQuarantineSize()>