intrusive_ptr not threadsafe

[kpu]

The reference counter is a bare size_t

marian-dev/src/common/intrusive_ptr.h

Lines 15 to 16 in 3daf4ee

	#define ENABLE_INTRUSIVE_PTR(type) \
	size_t references_{0}; \

and the pointer type is a bare pointer:

marian-dev/src/common/intrusive_ptr.h

Line 137 in 3daf4ee

T* ptr_;

If two threads destroy their references simultaneously, then both enter this function:

marian-dev/src/common/intrusive_ptr.h

Lines 23 to 28 in 3daf4ee

	inline friend void intrusivePtrRelease(type* x) { \
	if(x != 0 && --x->references_ == 0) { \
	delete x; \
	x = 0; \
	} \
	} \

It's possible both threads will read x != 0 and x->references_ == 2 then commit 1 back and leak the object.

Aside: this line doesn't do anything since it's in a function that takes type* x

marian-dev/src/common/intrusive_ptr.h

Line 26 in 3daf4ee

x = 0; \