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Editing: pycore_qsbr.h
// The QSBR APIs (quiescent state-based reclamation) provide a mechanism for // the free-threaded build to safely reclaim memory when there may be // concurrent accesses. // // Many operations in the free-threaded build are protected by locks. However, // in some cases, we want to allow reads to happen concurrently with updates. // In this case, we need to delay freeing ("reclaiming") any memory that may be // concurrently accessed by a reader. The QSBR APIs provide a way to do this. #ifndef Py_INTERNAL_QSBR_H #define Py_INTERNAL_QSBR_H #include <stdbool.h> #include <stdint.h> #include "pycore_lock.h" // PyMutex #ifdef __cplusplus extern "C" { #endif #ifndef Py_BUILD_CORE # error "this header requires Py_BUILD_CORE define" #endif // The shared write sequence is always odd and incremented by two. Detached // threads are indicated by a read sequence of zero. This avoids collisions // between the offline state and any valid sequence number even if the // sequences numbers wrap around. #define QSBR_OFFLINE 0 #define QSBR_INITIAL 1 #define QSBR_INCR 2 // Wrap-around safe comparison. This is a holdover from the FreeBSD // implementation, which uses 32-bit sequence numbers. We currently use 64-bit // sequence numbers, so wrap-around is unlikely. #define QSBR_LT(a, b) ((int64_t)((a)-(b)) < 0) #define QSBR_LEQ(a, b) ((int64_t)((a)-(b)) <= 0) struct _qsbr_shared; struct _PyThreadStateImpl; // forward declare to avoid circular dependency // Per-thread state struct _qsbr_thread_state { // Last observed write sequence (or 0 if detached) uint64_t seq; // Shared (per-interpreter) QSBR state struct _qsbr_shared *shared; // Thread state (or NULL) PyThreadState *tstate; // Number of held items added by this thread since the last write sequence // advance int deferred_count; // Estimate for the amount of memory that is held by this thread since // the last write sequence advance size_t deferred_memory; // Amount of memory in mimalloc pages deferred from collection. When // deferred, they are prevented from being used for a different size class // and in a different thread. size_t deferred_page_memory; // True if the deferred memory frees should be processed. bool should_process; // Is this thread state allocated? bool allocated; struct _qsbr_thread_state *freelist_next; }; // Padding to avoid false sharing struct _qsbr_pad { struct _qsbr_thread_state qsbr; char __padding[64 - sizeof(struct _qsbr_thread_state)]; }; // Per-interpreter state struct _qsbr_shared { // Write sequence: always odd, incremented by two uint64_t wr_seq; // Minimum observed read sequence of all QSBR thread states uint64_t rd_seq; // Array of QSBR thread states. struct _qsbr_pad *array; Py_ssize_t size; // Freelist of unused _qsbr_thread_states (protected by mutex) PyMutex mutex; struct _qsbr_thread_state *freelist; }; static inline uint64_t _Py_qsbr_shared_current(struct _qsbr_shared *shared) { return _Py_atomic_load_uint64_acquire(&shared->wr_seq); } // Reports a quiescent state: the caller no longer holds any pointer to shared // data not protected by locks or reference counts. static inline void _Py_qsbr_quiescent_state(struct _qsbr_thread_state *qsbr) { uint64_t seq = _Py_qsbr_shared_current(qsbr->shared); _Py_atomic_store_uint64_release(&qsbr->seq, seq); } // Have the read sequences advanced to the given goal? Like `_Py_qsbr_poll()`, // but does not perform a scan of threads. static inline bool _Py_qbsr_goal_reached(struct _qsbr_thread_state *qsbr, uint64_t goal) { uint64_t rd_seq = _Py_atomic_load_uint64(&qsbr->shared->rd_seq); return QSBR_LEQ(goal, rd_seq); } // Advance the write sequence and return the new goal. This should be called // after data is removed. The returned goal is used with `_Py_qsbr_poll()` to // determine when it is safe to reclaim (free) the memory. extern uint64_t _Py_qsbr_advance(struct _qsbr_shared *shared); // Return the next value for the write sequence (current plus the increment). extern uint64_t _Py_qsbr_shared_next(struct _qsbr_shared *shared); // Return true if deferred memory frees held by QSBR should be processed to // determine if they can be safely freed. static inline bool _Py_qsbr_should_process(struct _qsbr_thread_state *qsbr) { return qsbr->should_process; } // Have the read sequences advanced to the given goal? If this returns true, // it safe to reclaim any memory tagged with the goal (or earlier goal). extern bool _Py_qsbr_poll(struct _qsbr_thread_state *qsbr, uint64_t goal); // Called when thread attaches to interpreter extern void _Py_qsbr_attach(struct _qsbr_thread_state *qsbr); // Called when thread detaches from interpreter extern void _Py_qsbr_detach(struct _qsbr_thread_state *qsbr); // Reserves (allocates) a QSBR state and returns its index. extern Py_ssize_t _Py_qsbr_reserve(PyInterpreterState *interp); // Associates a PyThreadState with the QSBR state at the given index extern void _Py_qsbr_register(struct _PyThreadStateImpl *tstate, PyInterpreterState *interp, Py_ssize_t index); // Disassociates a PyThreadState from the QSBR state and frees the QSBR state. extern void _Py_qsbr_unregister(PyThreadState *tstate); extern void _Py_qsbr_fini(PyInterpreterState *interp); extern void _Py_qsbr_after_fork(struct _PyThreadStateImpl *tstate); #ifdef __cplusplus } #endif #endif /* !Py_INTERNAL_QSBR_H */
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