[scudo] Use a tryLock in secondary release to OS (#132827)

In the caching part of the secondary path, when about to try to release
memory to the OS, we always wait while acquiring the lock. However, if
multiple threads are attempting this at the same time, all other threads
will likely do nothing when the release call is made. Change the
algorithm to skip the release if there is another release in process.

Also, pull the lock outside the releaseOlderThan function. This is so
that in the store path, we use the tryLock and skip if another thread is
releasing. But in the path where a forced release call is being made,
that call will wait for release to complete which guarantees that all
entries are released when requested.

Author: cferris1000

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()>