[CK_TILE] Add async workspace prepare to FMHA BWD launcher

include/ck_tile/host/pinned_host_releaser.hpp

@@ -0,0 +1,77 @@
+// Copyright (c) Advanced Micro Devices, Inc., or its affiliates.
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <condition_variable>
+#include <hip/hip_runtime.h>
+#include <mutex>
+#include <queue>
+#include <thread>
+
+namespace ck_tile {
+
+// Defers hipHostFree off the HIP callback path. HIP callbacks hold runtime
+// locks, so calling hipHostFree (or any HIP API) from one deadlocks against
+// concurrent main-thread hipFree. enqueue() is HIP-API-free; a worker thread
+// drains the queue and calls hipHostFree. Use instance() for a process-wide
+// shared worker.
+class pinned_host_releaser
+{
+    std::mutex mtx_;
+    std::condition_variable cv_;
+    std::queue<void*> q_;
+    std::thread worker_;
+    bool stop_ = false;
+
+    void run()
+    {
+        for(;;)
+        {
+            void* p = nullptr;
+            {
+                std::unique_lock<std::mutex> lk(mtx_);
+                cv_.wait(lk, [&] { return stop_ || !q_.empty(); });
+                if(q_.empty())
+                    return; // stop_ && empty
+                p = q_.front();
+                q_.pop();
+            }
+            (void)hipHostFree(p);
+        }
+    }
+
+    public:
+    pinned_host_releaser() : worker_([this] { run(); }) {}
+
+    ~pinned_host_releaser()
+    {
+        {
+            std::lock_guard<std::mutex> lk(mtx_);
+            stop_ = true;
+        }
+        cv_.notify_all();
+        if(worker_.joinable())
+            worker_.join();
+    }
+
+    pinned_host_releaser(const pinned_host_releaser&)            = delete;
+    pinned_host_releaser& operator=(const pinned_host_releaser&) = delete;
+
+    static pinned_host_releaser& instance()
+    {
+        static pinned_host_releaser r;
+        return r;
+    }
+
+    void enqueue(void* p)
+    {
+        {
+            std::lock_guard<std::mutex> lk(mtx_);
+            q_.push(p);
+        }
+        cv_.notify_one();
+    }
+};
+
+} // namespace ck_tile