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[OpenMP] Fix node destruction race in __kmpc_omp_taskwait_deps_51 (#8…
…6130) The __kmpc_omp_taskwait_deps_51 allocates a kmp_depnode_t node on its stack, and there is currently a race condition where another thread might still be accessing that node after the function has returned and its stack frame was released. While the function does wait until the node's npredecessors count has reached zero before exiting, there is still a window where the function that last decremented the npredecessors count assumes the node is still accessible. For heap-allocated kmp_depnode_t nodes, this normally works via a separate ndeps count that only reaches zero at the point where no accesses to the node are expected at all; in fact, at this point the heap allocation will be freed. For this case of a stack-allocated kmp_depnode_t node, it therefore makes sense to similarly respect the ndeps count; we need to wait until this reaches 1 (not 0, because it is not heap-allocated so there's always one extra count to prevent it from being freed), before we can safely deallocate our stack frame. As this is expected to be a short race window of only a few instructions, it should be fine to just use a busy wait loop checking the ndeps count. Fixes: #85963
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