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gh-135871: Reload lock internal state while spinning in PyMutex_LockTimed #146064

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gh-135871: Reload lock internal state while spinning in PyMutex_LockTimed #146064

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15 changes: 15 additions & 0 deletions Python/lock.c
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Original file line number Diff line number Diff line change
Expand Up @@ -27,8 +27,10 @@ static const PyTime_t TIME_TO_BE_FAIR_NS = 1000*1000;
// enabled.
#if Py_GIL_DISABLED
static const int MAX_SPIN_COUNT = 40;
static const int RELOAD_SPIN_COUNT = 3;
#else
static const int MAX_SPIN_COUNT = 0;
static const int RELOAD_SPIN_COUNT = 1;
#endif

struct mutex_entry {
Expand Down Expand Up @@ -79,6 +81,16 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)
};

Py_ssize_t spin_count = 0;
#ifdef Py_GIL_DISABLED
// Using thread-id as a way of reducing contention further in the reload below.
// It adds a pseudo-random starting offset to the recurrence, so that threads
// are less likely to try and run compare-exchange at the same time.
// The lower bits of platform thread ids are likely to not be random,
// hence the right shift.
const Py_ssize_t tid = (Py_ssize_t) _Py_ThreadId() >> 12;
#else
const Py_ssize_t tid = 0;
#endif
for (;;) {
if ((v & _Py_LOCKED) == 0) {
// The lock is unlocked. Try to grab it.
Expand All @@ -92,6 +104,9 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)
// Spin for a bit.
_Py_yield();
spin_count++;
if (((spin_count + tid) & RELOAD_SPIN_COUNT) == 0) {
v = _Py_atomic_load_uint8_relaxed(&m->_bits);
}
continue;
}

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