Re: Atomic caching of smart pointers

On 9/26/2024 3:14 AM, Paavo Helde wrote:

On 26.09.2024 08:49, Chris M. Thomasson wrote:

On 9/17/2024 2:22 AM, Paavo Helde wrote:

On 17.09.2024 09:04, Chris M. Thomasson wrote:

On 9/16/2024 10:59 PM, Chris M. Thomasson wrote:

On 9/16/2024 10:54 PM, Paavo Helde wrote:

[...]

template<typename T>
class CachedAtomicPtr {
public:
     CachedAtomicPtr(): ptr_(nullptr) {}
>
     /// Store p in *this if *this is not yet assigned.
     /// Return pointer stored in *this, which can be \a p or not.
     Ptr<T> AssignIfNull(Ptr<T> p) {
         const T* other = nullptr;
         if (ptr_.compare_exchange_weak(other, p.get(), std::memory_order_release, std::memory_order_acquire)) {
             p->IncrementRefcount();
             return p;
         } else {
             // wrap in an extra smartptr (increments refcount)
             return Ptr<T>(other);
         }

>
^^^^^^^^^^^^^^^^^^
>
Is Ptr<T> an intrusive reference count? I assume it is.

>
Yes. Otherwise I could not generate new smartpointers from bare T*.
>
>
FYI, here is my current full compilable code together with a test harness (no relacy, could not get it working, so this just creates a number of threads which make use of the CachedAtomicPtr objects in parallel.
>
#include <cstddef>
#include <atomic>
#include <iostream>
#include <stdexcept>
#include <deque>
#include <mutex>
#include <thread>
#include <vector>
>
/// debug instrumentation
std::atomic<int> gAcount = 0, gBcount = 0, gCASFailureCount = 0;
/// program exit code
std::atomic<int> exitCode = EXIT_SUCCESS;
>
void Assert(bool x) {
     if (!x) {
         throw std::logic_error("Assert failed");
     }
}
>
class RefCountedBase {
public:
     RefCountedBase(): refcount_(0) {}
     RefCountedBase(const RefCountedBase&): refcount_(0) {}
     RefCountedBase(RefCountedBase&&) = delete;
     RefCountedBase& operator=(const RefCountedBase&) = delete;
     RefCountedBase& operator=(RefCountedBase&&) = delete;
>
     void Capture() const noexcept {
         ++refcount_;
     }
     void Release() const noexcept {
         if (--refcount_ == 0) {
             delete const_cast<RefCountedBase*>(this);
         }
     }
     virtual ~RefCountedBase() {}
>
>
private:
     mutable std::atomic<std::size_t> refcount_;
};
>
template<class T>
class Ptr {
public:
     Ptr(): ptr_(nullptr) {}
     explicit Ptr(const T* ptr): ptr_(ptr) { if (ptr_) { ptr_-  >Capture(); } }
     Ptr(const Ptr& b): ptr_(b.ptr_) { if (ptr_) { ptr_->Capture(); } }
     Ptr(Ptr&& b) noexcept: ptr_(b.ptr_) { b.ptr_ = nullptr; }
     ~Ptr() { if (ptr_) { ptr_->Release(); } }
     Ptr& operator=(const Ptr& b) {
         if (b.ptr_) { b.ptr_->Capture(); }
         if (ptr_) { ptr_->Release(); }
         ptr_ = b.ptr_;
         return *this;
     }
     Ptr& operator=(Ptr&& b) noexcept {
         if (ptr_) { ptr_->Release(); }
         ptr_ = b.ptr_;
         b.ptr_ = nullptr;
         return *this;
     }
     const T* operator->() const { return ptr_; }
     const T& operator*() const { return *ptr_; }
     explicit operator bool() const { return ptr_!=nullptr; }
     const T* get() const { return ptr_; }
private:
     mutable const T* ptr_;
};
>
template<typename T>
class CachedAtomicPtr {
public:
     CachedAtomicPtr(): ptr_(nullptr) {}
     /// Store p in *this if *this is not yet assigned.
     /// Return pointer stored in *this, which can be \a p or not.
     Ptr<T> AssignIfNull(Ptr<T> p) {
         const T* other = nullptr;
         if (ptr_.compare_exchange_strong(other, p.get(), std::memory_order_release, std::memory_order_acquire)) {
             p->Capture();

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

>
Only one thread should ever get here, right? It just installed the pointer p.get() into ptr_, right?

 Yes, that's the idea. The first thread which manages to install non-null pointer will increase the refcount, others will fail and their objects will be released when refcounts drop to zero.

[...]
Why do a Capture _after_ the pointer is atomically installed? Think of adding a reference in preparation for installation. If failed, it can decrement it. If it succeeded it leaves it alone.
<pseudo code>
___________________
shared refcount<foo>* g_foo = nullptr;
void thread_a()
{
     // initialized with two references
     refcount<foo> local = new refcount<foo>(2);
     refcount<foo>* shared = CAS_STRONG(&g_foo, nullptr, local);
     if (shared)
     {
        // another thread beat us to it.
        local.dec(); // we dec because we failed to install...
        // well, we have a reference to shared and local... :^)
     }
     else
     {
         // well, shared is nullptr so we were the first thread!
     }
}
___________________
Sorry for all the questions. ;^o