[gcc r13-10050] libstdc++: Fix memory leak in PSTL TBB backend [PR117276]

[Jonathan Wakely]

https://gcc.gnu.org/g:0caeca8cd77e23c8155e0f7c14de3aabac53b941

commit r13-10050-g0caeca8cd77e23c8155e0f7c14de3aabac53b941
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Sep 12 12:49:39 2025 +0100

    libstdc++: Fix memory leak in PSTL TBB backend [PR117276]
    
    Backport of upstream patch:
    https://github.com/uxlfoundation/oneDPL/pull/1589
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/117276
            * include/pstl/parallel_backend_tbb.h (__func_task::finalize):
            Make deallocation unconditional.
    
    (cherry picked from commit d8f1655a781a76f5c86b3545b181b2005e585d29)

Diff:
---
 libstdc++-v3/include/pstl/parallel_backend_tbb.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/pstl/parallel_backend_tbb.h b/libstdc++-v3/include/pstl/parallel_backend_tbb.h
index 413c7843bb8b..fe64bfc9a643 100644
--- a/libstdc++-v3/include/pstl/parallel_backend_tbb.h
+++ b/libstdc++-v3/include/pstl/parallel_backend_tbb.h
@@ -521,7 +521,7 @@ class __root_task
     friend class __func_task<_Func>;
 };
 
-#else  // TBB_INTERFACE_VERSION <= 12000
+#else  // TBB_INTERFACE_VERSION > 12000
 class __task : public tbb::detail::d1::task
 {
   protected:
@@ -656,10 +656,16 @@ class __func_task : public __task
 
         _PSTL_ASSERT(__parent != nullptr);
         _PSTL_ASSERT(__parent->_M_refcount.load(std::memory_order_relaxed) > 0);
-        if (--__parent->_M_refcount == 0)
+
+        auto __refcount = --__parent->_M_refcount;
+
+        // Placing the deallocation after the refcount decrement allows another thread to proceed with tree
+        // folding concurrently with this task cleanup.
+        __alloc.deallocate(this, *__ed);
+
+        if (__refcount == 0)
         {
             _PSTL_ASSERT(__next == nullptr);
-            __alloc.deallocate(this, *__ed);
             return __parent;
         }