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gh-118392: Update genrand_uint32 to use atomic operation #118393

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gh-118392: Update genrand_uint32 to use atomic operation #118393

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9 changes: 9 additions & 0 deletions Include/internal/pycore_pyatomic_ft_wrappers.h
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Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,8 @@ extern "C" {
_Py_atomic_load_uintptr_acquire(&value)
#define FT_ATOMIC_LOAD_PTR_RELAXED(value) \
_Py_atomic_load_ptr_relaxed(&value)
#define FT_ATOMIC_LOAD_INT(value) \
_Py_atomic_load_int(&value)
#define FT_ATOMIC_LOAD_UINT8(value) \
_Py_atomic_load_uint8(&value)
#define FT_ATOMIC_STORE_UINT8(value, new_value) \
Expand All @@ -49,6 +51,10 @@ extern "C" {
_Py_atomic_store_ssize_relaxed(&value, new_value)
#define FT_ATOMIC_STORE_UINT8_RELAXED(value, new_value) \
_Py_atomic_store_uint8_relaxed(&value, new_value)
#define FT_ATOMIC_STORE_INT(value, new_value) \
_Py_atomic_store_int(&value, new_value)
#define FT_ATMOIC_INT_INCREMENT(value) \
_Py_atomic_add_int(&value, 1)

#else
#define FT_ATOMIC_LOAD_PTR(value) value
Expand All @@ -60,13 +66,16 @@ extern "C" {
#define FT_ATOMIC_LOAD_PTR_ACQUIRE(value) value
#define FT_ATOMIC_LOAD_UINTPTR_ACQUIRE(value) value
#define FT_ATOMIC_LOAD_PTR_RELAXED(value) value
#define FT_ATOMIC_LOAD_INT(value) value
#define FT_ATOMIC_LOAD_UINT8(value) value
#define FT_ATOMIC_STORE_UINT8(value, new_value) value = new_value
#define FT_ATOMIC_STORE_PTR_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_STORE_PTR_RELEASE(value, new_value) value = new_value
#define FT_ATOMIC_STORE_UINTPTR_RELEASE(value, new_value) value = new_value
#define FT_ATOMIC_STORE_SSIZE_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_STORE_UINT8_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_STORE_INT(value, new_value) value = new_value
#define FT_ATMOIC_INT_INCREMENT(value) value++

#endif

Expand Down
14 changes: 7 additions & 7 deletions Modules/_randommodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -75,6 +75,7 @@
#include "pycore_modsupport.h" // _PyArg_NoKeywords()
#include "pycore_moduleobject.h" // _PyModule_GetState()
#include "pycore_pylifecycle.h" // _PyOS_URandomNonblock()
#include "pycore_pyatomic_ft_wrappers.h" // FT_LOAD_INT()

#ifdef HAVE_UNISTD_H
# include <unistd.h> // getpid()
Expand Down Expand Up @@ -139,7 +140,8 @@ genrand_uint32(RandomObject *self)
uint32_t *mt;

mt = self->state;
if (self->index >= N) { /* generate N words at one time */
int index = FT_ATOMIC_LOAD_INT(self->index);
if (index >= N) { /* generate N words at one time */
Comment on lines +143 to +144
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I don't think this will work well. This can end up returning the same generated numbers to different threads, which we don't want.

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@corona10 corona10 Apr 29, 2024 •
edited
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Hmm in that case, should we change the algorithm(thread-safe one) or fine-grained locking for this use case?

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I don't think there's a simple fix to the random module.

I commented on the issue. It'd be helpful to document the behavior and provide alternatives. For example, Java's java.util.Random includes the following comment:

Instances of java.util.Random are threadsafe. However, the concurrent use of the same java.util.Random instance across threads may encounter contention and consequent poor performance. Consider instead using ThreadLocalRandom in multithreaded designs.

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I commented on the issue. It'd be helpful to document the behavior and provide alternatives. For example, Java's java.util.Random includes the following comment:

Okay sounds acceptable :)

int kk;

for (kk=0;kk<N-M;kk++) {
Expand All @@ -153,10 +155,10 @@ genrand_uint32(RandomObject *self)
y = (mt[N-1]&UPPER_MASK)|(mt[0]&LOWER_MASK);
mt[N-1] = mt[M-1] ^ (y >> 1) ^ mag01[y & 0x1U];

self->index = 0;
FT_ATOMIC_STORE_INT(self->index, 0);
}

y = mt[self->index++];
y = mt[FT_ATMOIC_INT_INCREMENT(self->index)];
y ^= (y >> 11);
y ^= (y << 7) & 0x9d2c5680U;
y ^= (y << 15) & 0xefc60000U;
Expand All @@ -175,7 +177,6 @@ genrand_uint32(RandomObject *self)
*/

/*[clinic input]
@critical_section
_random.Random.random

self: self(type="RandomObject *")
Expand All @@ -185,7 +186,7 @@ random() -> x in the interval [0, 1).

static PyObject *
_random_Random_random_impl(RandomObject *self)
/*[clinic end generated code: output=117ff99ee53d755c input=26492e52d26e8b7b]*/
/*[clinic end generated code: output=117ff99ee53d755c input=afb2a59cbbb00349]*/
{
uint32_t a=genrand_uint32(self)>>5, b=genrand_uint32(self)>>6;
return PyFloat_FromDouble((a*67108864.0+b)*(1.0/9007199254740992.0));
Expand Down Expand Up @@ -491,7 +492,6 @@ _random_Random_setstate_impl(RandomObject *self, PyObject *state)
}

/*[clinic input]
@critical_section
_random.Random.getrandbits

self: self(type="RandomObject *")
Expand All @@ -503,7 +503,7 @@ getrandbits(k) -> x. Generates an int with k random bits.

static PyObject *
_random_Random_getrandbits_impl(RandomObject *self, int k)
/*[clinic end generated code: output=b402f82a2158887f input=87603cd60f79f730]*/
/*[clinic end generated code: output=b402f82a2158887f input=abfd108bf3c91945]*/
{
int i, words;
uint32_t r;
Expand Down
12 changes: 2 additions & 10 deletions Modules/clinic/_randommodule.c.h
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