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gh-145685: Remove `_PySeqLock` that is not used anymore (gh-155243) by maurycy · Pull Request #155243 · python/cpython · GitHub

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34 changes: 0 additions & 34 deletions Include/internal/pycore_lock.h
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Original file line number Diff line number Diff line change
Expand Up @@ -212,40 +212,6 @@ PyAPI_FUNC(void) _PyRWMutex_RUnlock(_PyRWMutex *rwmutex);
PyAPI_FUNC(void) _PyRWMutex_Lock(_PyRWMutex *rwmutex);
PyAPI_FUNC(void) _PyRWMutex_Unlock(_PyRWMutex *rwmutex);

// Similar to linux seqlock: https://en.wikipedia.org/wiki/Seqlock
// We use a sequence number to lock the writer, an even sequence means we're unlocked, an odd
// sequence means we're locked. Readers will read the sequence before attempting to read the
// underlying data and then read the sequence number again after reading the data. If the
// sequence has not changed the data is valid.
//
// Differs a little bit in that we use CAS on sequence as the lock, instead of a separate spin lock.
// The writer can also detect that the undelering data has not changed and abandon the write
// and restore the previous sequence.
typedef struct {
uint32_t sequence;
} _PySeqLock;

// Lock the sequence lock for the writer
PyAPI_FUNC(void) _PySeqLock_LockWrite(_PySeqLock *seqlock);

// Unlock the sequence lock and move to the next sequence number.
PyAPI_FUNC(void) _PySeqLock_UnlockWrite(_PySeqLock *seqlock);

// Abandon the current update indicating that no mutations have occurred
// and restore the previous sequence value.
PyAPI_FUNC(void) _PySeqLock_AbandonWrite(_PySeqLock *seqlock);

// Begin a read operation and return the current sequence number.
PyAPI_FUNC(uint32_t) _PySeqLock_BeginRead(_PySeqLock *seqlock);

// End the read operation and confirm that the sequence number has not changed.
// Returns 1 if the read was successful or 0 if the read should be retried.
PyAPI_FUNC(int) _PySeqLock_EndRead(_PySeqLock *seqlock, uint32_t previous);

// Check if the lock was held during a fork and clear the lock. Returns 1
// if the lock was held and any associated data should be cleared.
PyAPI_FUNC(int) _PySeqLock_AfterFork(_PySeqLock *seqlock);

#ifdef __cplusplus
}
#endif
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1 change: 0 additions & 1 deletion Objects/typeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,6 @@
#include "pycore_dict.h" // _PyDict_KeysSize()
#include "pycore_function.h" // _PyFunction_GetVersionForCurrentState()
#include "pycore_interpframe.h" // _PyInterpreterFrame
#include "pycore_lock.h" // _PySeqLock_*
#include "pycore_long.h" // _PyLong_IsNegative(), _PyLong_GetOne()
#include "pycore_memoryobject.h" // _PyMemoryView_FromBufferProc()
#include "pycore_modsupport.h" // _PyArg_NoKwnames()
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75 changes: 0 additions & 75 deletions Python/lock.c
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Original file line number Diff line number Diff line change
Expand Up @@ -574,81 +574,6 @@ _PyRWMutex_Unlock(_PyRWMutex *rwmutex)
}
}

#define SEQLOCK_IS_UPDATING(sequence) (sequence & 0x01)

void _PySeqLock_LockWrite(_PySeqLock *seqlock)
{
// lock by moving to an odd sequence number
uint32_t prev = _Py_atomic_load_uint32_relaxed(&seqlock->sequence);
while (1) {
if (SEQLOCK_IS_UPDATING(prev)) {
// Someone else is currently updating the cache
_Py_yield();
prev = _Py_atomic_load_uint32_relaxed(&seqlock->sequence);
}
else if (_Py_atomic_compare_exchange_uint32(&seqlock->sequence, &prev, prev + 1)) {
// We've locked the cache
_Py_atomic_fence_release();
break;
}
else {
_Py_yield();
}
}
}

void _PySeqLock_AbandonWrite(_PySeqLock *seqlock)
{
uint32_t new_seq = _Py_atomic_load_uint32_relaxed(&seqlock->sequence) - 1;
assert(!SEQLOCK_IS_UPDATING(new_seq));
_Py_atomic_store_uint32(&seqlock->sequence, new_seq);
}

void _PySeqLock_UnlockWrite(_PySeqLock *seqlock)
{
uint32_t new_seq = _Py_atomic_load_uint32_relaxed(&seqlock->sequence) + 1;
assert(!SEQLOCK_IS_UPDATING(new_seq));
_Py_atomic_store_uint32(&seqlock->sequence, new_seq);
}

uint32_t _PySeqLock_BeginRead(_PySeqLock *seqlock)
{
uint32_t sequence = _Py_atomic_load_uint32_acquire(&seqlock->sequence);
while (SEQLOCK_IS_UPDATING(sequence)) {
_Py_yield();
sequence = _Py_atomic_load_uint32_acquire(&seqlock->sequence);
}

return sequence;
}

int _PySeqLock_EndRead(_PySeqLock *seqlock, uint32_t previous)
{
// gh-121368: We need an explicit acquire fence here to ensure that
// this load of the sequence number is not reordered before any loads
// within the read lock.
_Py_atomic_fence_acquire();

if (_Py_atomic_load_uint32_relaxed(&seqlock->sequence) == previous) {
return 1;
}

_Py_yield();
return 0;
}

int _PySeqLock_AfterFork(_PySeqLock *seqlock)
{
// Synchronize again and validate that the entry hasn't been updated
// while we were readying the values.
if (SEQLOCK_IS_UPDATING(seqlock->sequence)) {
seqlock->sequence = 0;
return 1;
}

return 0;
}

#undef PyMutex_Lock
void
PyMutex_Lock(PyMutex *m)
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