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gh-115103: Implement delayed memory reclamation (QSBR) (#115180)
This adds a safe memory reclamation scheme based on FreeBSD's "GUS" and quiescent state based reclamation (QSBR). The API provides a mechanism for callers to detect when it is safe to free memory that may be concurrently accessed by readers.
1 parent 711f42d commit 5903190

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‎Doc/license.rst

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@@ -1095,3 +1095,35 @@ which is distributed under the MIT license::
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LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
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OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
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WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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Global Unbounded Sequences (GUS)
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--------------------------------
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The file :file:`Python/qsbr.c` is adapted from FreeBSD's "Global Unbounded
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Sequences" safe memory reclamation scheme in
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`subr_smr.c <https://github.com/freebsd/freebsd-src/blob/main/sys/kern/subr_smr.c>`_.
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The file is distributed under the 2-Clause BSD License::
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Copyright (c) 2019,2020 Jeffrey Roberson <jeff@FreeBSD.org>
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions
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are met:
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1. Redistributions of source code must retain the above copyright
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notice unmodified, this list of conditions, and the following
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disclaimer.
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2. Redistributions in binary form must reproduce the above copyright
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notice, this list of conditions and the following disclaimer in the
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documentation and/or other materials provided with the distribution.
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THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

‎Include/cpython/pyatomic.h

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@@ -475,6 +475,12 @@ _Py_atomic_store_int_release(int *obj, int value);
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static inline int
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_Py_atomic_load_int_acquire(const int *obj);
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static inline void
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_Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value);
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static inline uint64_t
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_Py_atomic_load_uint64_acquire(const uint64_t *obj);
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static inline uint32_t
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_Py_atomic_load_uint32_acquire(const uint32_t *obj);
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‎Include/cpython/pyatomic_gcc.h

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@@ -504,6 +504,14 @@ static inline int
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_Py_atomic_load_int_acquire(const int *obj)
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{ return __atomic_load_n(obj, __ATOMIC_ACQUIRE); }
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static inline void
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_Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value)
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{ __atomic_store_n(obj, value, __ATOMIC_RELEASE); }
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static inline uint64_t
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_Py_atomic_load_uint64_acquire(const uint64_t *obj)
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{ return __atomic_load_n(obj, __ATOMIC_ACQUIRE); }
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static inline uint32_t
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_Py_atomic_load_uint32_acquire(const uint32_t *obj)
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{ return __atomic_load_n(obj, __ATOMIC_ACQUIRE); }

‎Include/cpython/pyatomic_msc.h

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@@ -952,13 +952,39 @@ _Py_atomic_load_int_acquire(const int *obj)
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#endif
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}
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static inline void
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_Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value)
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{
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#if defined(_M_X64) || defined(_M_IX86)
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*(uint64_t volatile *)obj = value;
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#elif defined(_M_ARM64)
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_Py_atomic_ASSERT_ARG_TYPE(unsigned __int64);
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__stlr64((unsigned __int64 volatile *)obj, (unsigned __int64)value);
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#else
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# error "no implementation of _Py_atomic_store_uint64_release"
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#endif
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}
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static inline uint64_t
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_Py_atomic_load_uint64_acquire(const uint64_t *obj)
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{
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#if defined(_M_X64) || defined(_M_IX86)
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return *(uint64_t volatile *)obj;
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#elif defined(_M_ARM64)
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_Py_atomic_ASSERT_ARG_TYPE(__int64);
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return (uint64_t)__ldar64((unsigned __int64 volatile *)obj);
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#else
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# error "no implementation of _Py_atomic_load_uint64_acquire"
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#endif
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}
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static inline uint32_t
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_Py_atomic_load_uint32_acquire(const uint32_t *obj)
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{
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#if defined(_M_X64) || defined(_M_IX86)
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return *(uint32_t volatile *)obj;
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#elif defined(_M_ARM64)
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return (int)__ldar32((uint32_t volatile *)obj);
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return (uint32_t)__ldar32((uint32_t volatile *)obj);
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#else
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# error "no implementation of _Py_atomic_load_uint32_acquire"
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#endif

‎Include/cpython/pyatomic_std.h

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@@ -887,6 +887,22 @@ _Py_atomic_load_int_acquire(const int *obj)
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memory_order_acquire);
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}
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static inline void
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_Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value)
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{
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_Py_USING_STD;
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atomic_store_explicit((_Atomic(uint64_t)*)obj, value,
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memory_order_release);
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}
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static inline uint64_t
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_Py_atomic_load_uint64_acquire(const uint64_t *obj)
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{
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_Py_USING_STD;
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return atomic_load_explicit((const _Atomic(uint64_t)*)obj,
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memory_order_acquire);
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}
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static inline uint32_t
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_Py_atomic_load_uint32_acquire(const uint32_t *obj)
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{

‎Include/internal/pycore_interp.h

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@@ -30,6 +30,7 @@ extern "C" {
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#include "pycore_mimalloc.h" // struct _mimalloc_interp_state
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#include "pycore_object_state.h" // struct _py_object_state
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#include "pycore_obmalloc.h" // struct _obmalloc_state
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#include "pycore_qsbr.h" // struct _qsbr_state
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#include "pycore_tstate.h" // _PyThreadStateImpl
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#include "pycore_tuple.h" // struct _Py_tuple_state
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#include "pycore_typeobject.h" // struct types_state
@@ -197,6 +198,7 @@ struct _is {
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struct _warnings_runtime_state warnings;
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struct atexit_state atexit;
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struct _stoptheworld_state stoptheworld;
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struct _qsbr_shared qsbr;
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#if defined(Py_GIL_DISABLED)
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struct _mimalloc_interp_state mimalloc;

‎Include/internal/pycore_qsbr.h

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@@ -0,0 +1,139 @@
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// The QSBR APIs (quiescent state-based reclamation) provide a mechanism for
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// the free-threaded build to safely reclaim memory when there may be
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// concurrent accesses.
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//
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// Many operations in the free-threaded build are protected by locks. However,
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// in some cases, we want to allow reads to happen concurrently with updates.
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// In this case, we need to delay freeing ("reclaiming") any memory that may be
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// concurrently accessed by a reader. The QSBR APIs provide a way to do this.
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#ifndef Py_INTERNAL_QSBR_H
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#define Py_INTERNAL_QSBR_H
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#include <stdbool.h>
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#include <stdint.h>
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#include "pycore_lock.h" // PyMutex
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#ifdef __cplusplus
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extern "C" {
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#endif
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#ifndef Py_BUILD_CORE
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# error "this header requires Py_BUILD_CORE define"
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#endif
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// The shared write sequence is always odd and incremented by two. Detached
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// threads are indicated by a read sequence of zero. This avoids collisions
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// between the offline state and any valid sequence number even if the
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// sequences numbers wrap around.
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#define QSBR_OFFLINE 0
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#define QSBR_INITIAL 1
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#define QSBR_INCR 2
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struct _qsbr_shared;
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struct _PyThreadStateImpl; // forward declare to avoid circular dependency
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// Per-thread state
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struct _qsbr_thread_state {
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// Last observed write sequence (or 0 if detached)
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uint64_t seq;
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// Shared (per-interpreter) QSBR state
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struct _qsbr_shared *shared;
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// Thread state (or NULL)
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PyThreadState *tstate;
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// Used to defer advancing write sequence a fixed number of times
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int deferrals;
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// Is this thread state allocated?
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bool allocated;
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struct _qsbr_thread_state *freelist_next;
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};
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// Padding to avoid false sharing
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struct _qsbr_pad {
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struct _qsbr_thread_state qsbr;
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char __padding[64 - sizeof(struct _qsbr_thread_state)];
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};
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// Per-interpreter state
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struct _qsbr_shared {
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// Write sequence: always odd, incremented by two
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uint64_t wr_seq;
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// Minimum observed read sequence of all QSBR thread states
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uint64_t rd_seq;
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// Array of QSBR thread states.
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struct _qsbr_pad *array;
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Py_ssize_t size;
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// Freelist of unused _qsbr_thread_states (protected by mutex)
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PyMutex mutex;
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struct _qsbr_thread_state *freelist;
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};
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static inline uint64_t
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_Py_qsbr_shared_current(struct _qsbr_shared *shared)
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{
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return _Py_atomic_load_uint64_acquire(&shared->wr_seq);
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}
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// Reports a quiescent state: the caller no longer holds any pointer to shared
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// data not protected by locks or reference counts.
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static inline void
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_Py_qsbr_quiescent_state(struct _qsbr_thread_state *qsbr)
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{
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uint64_t seq = _Py_qsbr_shared_current(qsbr->shared);
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_Py_atomic_store_uint64_release(&qsbr->seq, seq);
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}
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// Advance the write sequence and return the new goal. This should be called
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// after data is removed. The returned goal is used with `_Py_qsbr_poll()` to
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// determine when it is safe to reclaim (free) the memory.
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extern uint64_t
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_Py_qsbr_advance(struct _qsbr_shared *shared);
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// Batches requests to advance the write sequence. This advances the write
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// sequence every N calls, which reduces overhead but increases time to
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// reclamation. Returns the new goal.
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extern uint64_t
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_Py_qsbr_deferred_advance(struct _qsbr_thread_state *qsbr);
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// Have the read sequences advanced to the given goal? If this returns true,
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// it safe to reclaim any memory tagged with the goal (or earlier goal).
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extern bool
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_Py_qsbr_poll(struct _qsbr_thread_state *qsbr, uint64_t goal);
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// Called when thread attaches to interpreter
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extern void
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_Py_qsbr_attach(struct _qsbr_thread_state *qsbr);
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// Called when thread detaches from interpreter
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extern void
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_Py_qsbr_detach(struct _qsbr_thread_state *qsbr);
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// Reserves (allocates) a QSBR state and returns its index.
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extern Py_ssize_t
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_Py_qsbr_reserve(PyInterpreterState *interp);
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// Associates a PyThreadState with the QSBR state at the given index
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extern void
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_Py_qsbr_register(struct _PyThreadStateImpl *tstate,
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PyInterpreterState *interp, Py_ssize_t index);
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// Disassociates a PyThreadState from the QSBR state and frees the QSBR state.
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extern void
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_Py_qsbr_unregister(struct _PyThreadStateImpl *tstate);
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extern void
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_Py_qsbr_fini(PyInterpreterState *interp);
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extern void
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_Py_qsbr_after_fork(struct _PyThreadStateImpl *tstate);
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#ifdef __cplusplus
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}
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#endif
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#endif /* !Py_INTERNAL_QSBR_H */

‎Include/internal/pycore_runtime_init.h

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@@ -17,6 +17,7 @@ extern "C" {
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#include "pycore_pyhash.h" // pyhash_state_INIT
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#include "pycore_pymem_init.h" // _pymem_allocators_standard_INIT
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#include "pycore_pythread.h" // _pythread_RUNTIME_INIT
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#include "pycore_qsbr.h" // QSBR_INITIAL
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#include "pycore_runtime_init_generated.h" // _Py_bytes_characters_INIT
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#include "pycore_signal.h" // _signals_RUNTIME_INIT
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#include "pycore_tracemalloc.h" // _tracemalloc_runtime_state_INIT
@@ -169,6 +170,10 @@ extern PyTypeObject _PyExc_MemoryError;
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{ .threshold = 10, }, \
170171
}, \
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}, \
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.qsbr = { \
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.wr_seq = QSBR_INITIAL, \
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.rd_seq = QSBR_INITIAL, \
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}, \
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.dtoa = _dtoa_state_INIT(&(INTERP)), \
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.dict_state = _dict_state_INIT, \
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.func_state = { \

‎Include/internal/pycore_tstate.h

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# error "this header requires Py_BUILD_CORE define"
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#endif
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#include "pycore_brc.h" // struct _brc_thread_state
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#include "pycore_freelist.h" // struct _Py_freelist_state
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#include "pycore_mimalloc.h" // struct _mimalloc_thread_state
13-
#include "pycore_brc.h" // struct _brc_thread_state
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#include "pycore_qsbr.h" // struct qsbr
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1617
static inline void
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2728
// semi-public fields are in PyThreadState.
2829
PyThreadState base;
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struct _qsbr_thread_state *qsbr; // only used by free-threaded build
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#ifdef Py_GIL_DISABLED
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struct _gc_thread_state gc;
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struct _mimalloc_thread_state mimalloc;

‎Makefile.pre.in

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Python/pystate.o \
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Python/pythonrun.o \
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Python/pytime.o \
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Python/qsbr.o \
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Python/bootstrap_hash.o \
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Python/specialize.o \
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Python/structmember.o \
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$(srcdir)/Include/internal/pycore_pystats.h \
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$(srcdir)/Include/internal/pycore_pythonrun.h \
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$(srcdir)/Include/internal/pycore_pythread.h \
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$(srcdir)/Include/internal/pycore_qsbr.h \
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$(srcdir)/Include/internal/pycore_range.h \
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$(srcdir)/Include/internal/pycore_runtime.h \
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$(srcdir)/Include/internal/pycore_runtime_init.h \

‎Modules/posixmodule.c

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#ifdef Py_GIL_DISABLED
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_Py_brc_after_fork(tstate->interp);
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_Py_qsbr_after_fork((_PyThreadStateImpl *)tstate);
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#endif
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status = _PyEval_ReInitThreads(tstate);

‎PCbuild/_freeze_module.vcxproj

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<ClCompile Include="..\Python\pythonrun.c" />
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<ClCompile Include="..\Python\Python-tokenize.c" />
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<ClCompile Include="..\Python\pytime.c" />
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<ClCompile Include="..\Python\qsbr.c" />
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<ClCompile Include="..\Python\specialize.c" />
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<ClCompile Include="..\Python\structmember.c" />
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<ClCompile Include="..\Python\suggestions.c" />

‎PCbuild/_freeze_module.vcxproj.filters

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<ClCompile Include="..\Python\pytime.c">
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<Filter>Source Files</Filter>
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</ClCompile>
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<ClCompile Include="..\Python\qsbr.c">
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<Filter>Source Files</Filter>
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</ClCompile>
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<ClCompile Include="..\Objects\rangeobject.c">
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<Filter>Source Files</Filter>
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</ClCompile>

‎PCbuild/pythoncore.vcxproj

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<ClInclude Include="..\Include\internal\pycore_pystats.h" />
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<ClInclude Include="..\Include\internal\pycore_pythonrun.h" />
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<ClInclude Include="..\Include\internal\pycore_pythread.h" />
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<ClInclude Include="..\Include\internal\pycore_qsbr.h" />
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<ClInclude Include="..\Include\internal\pycore_range.h" />
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<ClInclude Include="..\Include\internal\pycore_runtime.h" />
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<ClInclude Include="..\Include\internal\pycore_runtime_init.h" />
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<ClCompile Include="..\Python\pystrcmp.c" />
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<ClCompile Include="..\Python\pystrhex.c" />
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<ClCompile Include="..\Python\pystrtod.c" />
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<ClCompile Include="..\Python\qsbr.c" />
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<ClCompile Include="..\Python\dtoa.c" />
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<ClCompile Include="..\Python\Python-ast.c" />
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<ClCompile Include="..\Python\Python-tokenize.c" />

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