[mlir] Fix race condition introduced in ThreadLocalCache

Okay, so an apparently not-so-rare crash could occur if the
`perInstanceState` point got re-allocated to the same pointer as before.
All the values have been destroyed, so the TLC is left with dangling
pointers, but then the same `ValueT *` is pulled out of the TLC and
dereferenced, leading to a crash.

I suppose the purpose of the `weak_ptr` was that it would get reset to a
default state when the `perInstanceState` shared pointer got destryoed
(its reference count would only ever be 1, very briefly 2 when it gets
aliased to a `ValueT *` but then assigned to a `weak_ptr`).

Basically, there are circular references between TLC instances and
`perInstanceState` instances and we have to ensure there are no dangling
references.

1. Ensure the TLC entries are reset to a valid default state if the TLC
   (i.e. owning thread) lives longer than the `perInstanceState`.
    a. This is currently achieved by storing `weak_ptr` in the TLC.
2. If `perInstanceState` lives longer than the TLC, it cannot contain
   dangling references to entries in destroyed TLCs.
    a. This is not currently the case.
3. If both are being destroyed at the same time, we cannot race.
    a. The destructors are synchronized because the TLC destructor locks
       `weak_ptr` while it is destructing, preventing the owning
       `perInstanceState` of the entry from destructing. If
       `perInstanceState` got destructed first, the `weak_ptr` lock would
        fail.

And

4. Ensure `get` in the common (initialized) case is as fast as possible
   (no atomics).

We need to change the TLC to store a `ValueT **` so that it can be
shared with entries owned by `perInstanceState` and written to null when
they are destroyed. However, this is no longer something synchronized by
an atomic, meaning that (2) becomes a problem. This is fine because when
TLC destructs, we remove the entries from `perInstanceState` that could
reference the TLC entries.

This patch shows the same perf gain as before but hopefully without the
bug.

Author: Mogball

mlir/include/mlir/Support/ThreadLocalCache.h

@@ -16,7 +16,6 @@
 
 #include "mlir/Support/LLVM.h"
 #include "llvm/ADT/DenseMap.h"
-#include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/Mutex.h"
 
 namespace mlir {
@@ -25,28 +24,91 @@ namespace mlir {
 /// cache has very large lock contention.
 template <typename ValueT>
 class ThreadLocalCache {
+  struct PerInstanceState;
+
+  /// The "observer" is owned by a thread-local cache instance. It is
+  /// constructed the first time a `ThreadLocalCache` instance is accessed by a
+  /// thread, unless `perInstanceState` happens to get re-allocated to the same
+  /// address as a previous one. This class is destructed the thread in which
+  /// the `thread_local` cache lives is destroyed.
+  ///
+  /// This class is called the "observer" because while values cached in
+  /// thread-local caches are owned by `PerInstanceState`, a reference is stored
+  /// via this class in the TLC. With a double pointer, it knows when the
+  /// referenced value has been destroyed.
+  struct Observer {
+    Observer() : ptr(std::make_unique<ValueT *>(nullptr)) {}
+
+    /// This is the double pointer, explicitly allocated because we need to keep
+    /// the address stable if the TLC map re-allocates. It is owned by the
+    /// observer and shared with the value owner.
+    std::unique_ptr<ValueT *> ptr;
+    /// Because `Owner` living inside `PerInstanceState` contains a reference to
+    /// the double pointer, and livkewise this class contains a reference to the
+    /// value, we need to synchronize destruction of the TLC and the
+    /// `PerInstanceState` to avoid racing. This weak pointer is acquired during
+    /// TLC destruction if the `PerInstanceState` hasn't entered its destructor
+    /// yet, and prevents it from happening.
+    std::weak_ptr<PerInstanceState> keepalive;
+  };
+
+  /// This struct owns the cache entries. It contains a reference back to the
+  /// reference inside the cache so that it can be written to null to indicate
+  /// that the cache entry is invalidated. It needs to do this because
+  /// `perInstanceState` could get re-allocated to the same pointer and we don't
+  /// remove entries from the TLC when it is deallocated. Thus, we have to reset
+  /// the TLC entries to a starting state in case the `ThreadLocalCache` lives
+  /// shorter than the threads.
+  struct Owner {
+    /// Save a pointer to the reference and write it to the newly created entry.
+    Owner(Observer &observer)
+        : value(std::make_unique<ValueT>()), ptrRef(observer.ptr.get()) {
+      *ptrRef = value.get();
+    }
+    /// Upon destruction, reset it to nullptr so the next time the cache is hit
+    /// with the same pointer, it will restart in a valid state.
+    ~Owner() {
+      if (ptrRef)
+        *ptrRef = nullptr;
+    }
+
+    Owner(Owner &&other) : value(std::move(other.value)), ptrRef(other.ptrRef) {
+      other.ptrRef = nullptr;
+    }
+    Owner &operator=(Owner &&other) {
+      value = std::move(other.value);
+      ptrRef = other.ptrRef;
+      other.ptrRef = nullptr;
+      return *this;
+    }
+
+    std::unique_ptr<ValueT> value;
+    ValueT **ptrRef;
+  };
+
   // Keep a separate shared_ptr protected state that can be acquired atomically
   // instead of using shared_ptr's for each value. This avoids a problem
   // where the instance shared_ptr is locked() successfully, and then the
   // ThreadLocalCache gets destroyed before remove() can be called successfully.
   struct PerInstanceState {
-    /// Remove the given value entry. This is generally called when a thread
-    /// local cache is destructing.
+    /// Remove the given value entry. This is called when a thread local cache
+    /// is destructing but still contains references to values owned by the
+    /// `PerInstanceState`. Removal is required because it prevents writeback to
+    /// a pointer that was deallocated.
     void remove(ValueT *value) {
       // Erase the found value directly, because it is guaranteed to be in the
       // list.
       llvm::sys::SmartScopedLock<true> threadInstanceLock(instanceMutex);
-      auto it =
-          llvm::find_if(instances, [&](std::unique_ptr<ValueT> &instance) {
-            return instance.get() == value;
-          });
+      auto it = llvm::find_if(instances, [&](Owner &instance) {
+        return instance.value.get() == value;
+      });
       assert(it != instances.end() && "expected value to exist in cache");
       instances.erase(it);
     }
 
     /// Owning pointers to all of the values that have been constructed for this
     /// object in the static cache.
-    SmallVector<std::unique_ptr<ValueT>, 1> instances;
+    SmallVector<Owner, 1> instances;
 
     /// A mutex used when a new thread instance has been added to the cache for
     /// this object.
@@ -57,22 +119,22 @@ class ThreadLocalCache {
   /// instance of the non-static cache and a weak reference to an instance of
   /// ValueT. We use a weak reference here so that the object can be destroyed
   /// without needing to lock access to the cache itself.
-  struct CacheType
-      : public llvm::SmallDenseMap<PerInstanceState *,
-                                   std::pair<std::weak_ptr<ValueT>, ValueT *>> {
+  struct CacheType : public llvm::SmallDenseMap<PerInstanceState *, Observer> {
     ~CacheType() {
-      // Remove the values of this cache that haven't already expired.
-      for (auto &it : *this)
-        if (std::shared_ptr<ValueT> value = it.second.first.lock())
-          it.first->remove(value.get());
+      // Remove the values of this cache that haven't already expired. This is
+      // required because if we don't remove them, they will contain a reference
+      // back to the data here that is being destroyed.
+      for (auto &[instance, observer] : *this)
+        if (std::shared_ptr<PerInstanceState> state = observer.keepalive.lock())
+          state->remove(*observer.ptr);
     }
 
     /// Clear out any unused entries within the map. This method is not
     /// thread-safe, and should only be called by the same thread as the cache.
     void clearExpiredEntries() {
       for (auto it = this->begin(), e = this->end(); it != e;) {
         auto curIt = it++;
-        if (curIt->second.first.expired())
+        if (!*curIt->second.ptr)
           this->erase(curIt);
       }
     }
@@ -89,27 +151,23 @@ class ThreadLocalCache {
   ValueT &get() {
     // Check for an already existing instance for this thread.
     CacheType &staticCache = getStaticCache();
-    std::pair<std::weak_ptr<ValueT>, ValueT *> &threadInstance =
-        staticCache[perInstanceState.get()];
-    if (ValueT *value = threadInstance.second)
+    Observer &threadInstance = staticCache[perInstanceState.get()];
+    if (ValueT *value = *threadInstance.ptr)
       return *value;
 
     // Otherwise, create a new instance for this thread.
     {
       llvm::sys::SmartScopedLock<true> threadInstanceLock(
           perInstanceState->instanceMutex);
-      threadInstance.second =
-          perInstanceState->instances.emplace_back(std::make_unique<ValueT>())
-              .get();
+      perInstanceState->instances.emplace_back(threadInstance);
     }
-    threadInstance.first =
-        std::shared_ptr<ValueT>(perInstanceState, threadInstance.second);
+    threadInstance.keepalive = perInstanceState;
 
     // Before returning the new instance, take the chance to clear out any used
     // entries in the static map. The cache is only cleared within the same
     // thread to remove the need to lock the cache itself.
     staticCache.clearExpiredEntries();
-    return *threadInstance.second;
+    return **threadInstance.ptr;
   }
   ValueT &operator*() { return get(); }
   ValueT *operator->() { return &get(); }