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Biased reference counting maintains two refcount fields in each object: `ob_ref_local` and `ob_ref_shared`. The true refcount is the sum of these two fields. In some cases, when refcounting operations are split across threads, the ob_ref_shared field can be negative (although the total refcount must be at least zero). In this case, the thread that decremented the refcount requests that the owning thread give up ownership and merge the refcount fields.
75 lines
2.1 KiB
C
75 lines
2.1 KiB
C
#ifndef Py_INTERNAL_BRC_H
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#define Py_INTERNAL_BRC_H
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#include <stdint.h>
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#include "pycore_llist.h" // struct llist_node
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#include "pycore_lock.h" // PyMutex
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#include "pycore_object_stack.h" // _PyObjectStack
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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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#ifdef Py_GIL_DISABLED
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// Prime number to avoid correlations with memory addresses.
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#define _Py_BRC_NUM_BUCKETS 257
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// Hash table bucket
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struct _brc_bucket {
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// Mutex protects both the bucket and thread state queues in this bucket.
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PyMutex mutex;
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// Linked list of _PyThreadStateImpl objects hashed to this bucket.
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struct llist_node root;
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};
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// Per-interpreter biased reference counting state
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struct _brc_state {
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// Hash table of thread states by thread-id. Thread states within a bucket
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// are chained using a doubly-linked list.
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struct _brc_bucket table[_Py_BRC_NUM_BUCKETS];
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};
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// Per-thread biased reference counting state
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struct _brc_thread_state {
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// Linked-list of thread states per hash bucket
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struct llist_node bucket_node;
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// Thread-id as determined by _PyThread_Id()
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uintptr_t tid;
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// Objects with refcounts to be merged (protected by bucket mutex)
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_PyObjectStack objects_to_merge;
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// Local stack of objects to be merged (not accessed by other threads)
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_PyObjectStack local_objects_to_merge;
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};
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// Initialize/finalize the per-thread biased reference counting state
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void _Py_brc_init_thread(PyThreadState *tstate);
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void _Py_brc_remove_thread(PyThreadState *tstate);
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// Initialize per-interpreter state
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void _Py_brc_init_state(PyInterpreterState *interp);
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void _Py_brc_after_fork(PyInterpreterState *interp);
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// Enqueues an object to be merged by it's owning thread (tid). This
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// steals a reference to the object.
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void _Py_brc_queue_object(PyObject *ob);
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// Merge the refcounts of queued objects for the current thread.
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void _Py_brc_merge_refcounts(PyThreadState *tstate);
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#endif /* Py_GIL_DISABLED */
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#ifdef __cplusplus
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}
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#endif
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#endif /* !Py_INTERNAL_BRC_H */
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