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Possible race condition in atomic_add_double #3480

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Possible race condition in atomic_add_double #3480

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24 changes: 14 additions & 10 deletions stan/math/opencl/kernels/device_functions/atomic_add_double.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -18,13 +18,14 @@ static constexpr const char *atomic_add_double_device_function
/** \ingroup opencl_kernels
* Atomically add to a double value.
*
* Code is taken from:
* Adapted from:
* https://stackoverflow.com/questions/31863587/atomic-operations-with-double-opencl
*
* @param val pointer to value to add to
* @param delta value to add
*/
void atomic_add_double(__global double *val, double delta) {
volatile __global ulong *p = (volatile __global ulong *)val;
union {
double f;
ulong i;
Expand All @@ -33,23 +34,25 @@ static constexpr const char *atomic_add_double_device_function
double f;
ulong i;
} new_val;
// atomic read: a compare-and-swap that never changes the cell
ulong current = atom_cmpxchg(p, 0UL, 0UL);
do {
old_val.f = *val;
old_val.i = current;
new_val.f = old_val.f + delta;
} while (atom_cmpxchg((volatile __global ulong *)val, old_val.i,
new_val.i)
!= old_val.i);
current = atom_cmpxchg(p, old_val.i, new_val.i);
} while (current != old_val.i);
}
/** \ingroup opencl_kernels
* Atomically add to a local double value.
*
* Code is taken from:
* Adapted from:
* https://stackoverflow.com/questions/31863587/atomic-operations-with-double-opencl
*
* @param val pointer to value to add to
* @param delta value to add
*/
void local_atomic_add_double(__local double *val, double delta) {
volatile __local ulong *p = (volatile __local ulong *)val;
union {
double f;
ulong i;
Expand All @@ -58,12 +61,13 @@ static constexpr const char *atomic_add_double_device_function
double f;
ulong i;
} new_val;
// atomic read: a compare-and-swap that never changes the cell
ulong current = atom_cmpxchg(p, 0UL, 0UL);
do {
old_val.f = *val;
old_val.i = current;
new_val.f = old_val.f + delta;
} while (atom_cmpxchg((volatile __local ulong *)val, old_val.i,
new_val.i)
!= old_val.i);
current = atom_cmpxchg(p, old_val.i, new_val.i);
} while (current != old_val.i);
}
// \cond
) "\n#endif\n"; // NOLINT
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