Revert "[mlir] Fix race condition introduced in ThreadLocalCache (#93…

[kiranchandramohan]

…280)"

This reverts commit 6977bfb.

[llvmbot]

@llvm/pr-subscribers-mlir

@llvm/pr-subscribers-mlir-core

Author: Kiran Chandramohan (kiranchandramohan)

Changes

…280)"

This reverts commit 6977bfb.

---

Full diff: https://github.com/llvm/llvm-project/pull/93290.diff

1 Files Affected:

• (modified) mlir/include/mlir/Support/ThreadLocalCache.h (+25-72)

diff --git a/mlir/include/mlir/Support/ThreadLocalCache.h b/mlir/include/mlir/Support/ThreadLocalCache.h
index fe6c6fa3cf6bd..d19257bf6e25e 100644
--- a/mlir/include/mlir/Support/ThreadLocalCache.h
+++ b/mlir/include/mlir/Support/ThreadLocalCache.h
@@ -16,6 +16,7 @@
 
 #include "mlir/Support/LLVM.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/Mutex.h"
 
 namespace mlir {
@@ -24,80 +25,28 @@ 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 {
-    /// 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::shared_ptr<ValueT *> ptr = std::make_shared<ValueT *>(nullptr);
-    /// 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) {
-      *observer.ptr = value.get();
-    }
-    ~Owner() {
-      if (std::shared_ptr<ValueT *> ptr = ptrRef.lock())
-        *ptr = nullptr;
-    }
-
-    Owner(Owner &&) = default;
-    Owner &operator=(Owner &&) = default;
-
-    std::unique_ptr<ValueT> value;
-    std::weak_ptr<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 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.
+    /// Remove the given value entry. This is generally called when a thread
+    /// local cache is destructing.
     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, [&](Owner &instance) {
-        return instance.value.get() == value;
-      });
+      auto it =
+          llvm::find_if(instances, [&](std::unique_ptr<ValueT> &instance) {
+            return instance.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<Owner, 1> instances;
+    SmallVector<std::unique_ptr<ValueT>, 1> instances;
 
     /// A mutex used when a new thread instance has been added to the cache for
     /// this object.
@@ -108,14 +57,14 @@ 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 *, Observer> {
+  struct CacheType
+      : public llvm::SmallDenseMap<PerInstanceState *,
+                                   std::pair<std::weak_ptr<ValueT>, ValueT *>> {
     ~CacheType() {
-      // 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);
+      // 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());
     }
 
     /// Clear out any unused entries within the map. This method is not
@@ -123,7 +72,7 @@ class ThreadLocalCache {
     void clearExpiredEntries() {
       for (auto it = this->begin(), e = this->end(); it != e;) {
         auto curIt = it++;
-        if (!*curIt->second.ptr)
+        if (curIt->second.first.expired())
           this->erase(curIt);
       }
     }
@@ -140,23 +89,27 @@ class ThreadLocalCache {
   ValueT &get() {
     // Check for an already existing instance for this thread.
     CacheType &staticCache = getStaticCache();
-    Observer &threadInstance = staticCache[perInstanceState.get()];
-    if (ValueT *value = *threadInstance.ptr)
+    std::pair<std::weak_ptr<ValueT>, ValueT *> &threadInstance =
+        staticCache[perInstanceState.get()];
+    if (ValueT *value = threadInstance.second)
       return *value;
 
     // Otherwise, create a new instance for this thread.
     {
       llvm::sys::SmartScopedLock<true> threadInstanceLock(
           perInstanceState->instanceMutex);
-      perInstanceState->instances.emplace_back(threadInstance);
+      threadInstance.second =
+          perInstanceState->instances.emplace_back(std::make_unique<ValueT>())
+              .get();
     }
-    threadInstance.keepalive = perInstanceState;
+    threadInstance.first =
+        std::shared_ptr<ValueT>(perInstanceState, threadInstance.second);
 
     // 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.ptr;
+    return *threadInstance.second;
   }
   ValueT &operator*() { return get(); }
   ValueT *operator->() { return &get(); }

[joker-eph]

Thanks for the revert!

2 nitpicks for the future:

1. Please always provide the revert reason in the description.

2. Be mindful of GitHub wrapping of the title, it should be undone by editing the description and title when opening the PR.

Here is what your commit looks like in the history right now:

[kiranchandramohan]

Thanks for the revert!

2 nitpicks for the future:

1. Please always provide the revert reason in the description.
2. Be mindful of GitHub wrapping of the title, it should be undone by editing the description and title when opening the PR.

Here is what your commit looks like in the history right now:

Sure. Sorry for the inconvenience.