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3e60c6f723
Declare the toc_nentry field as uint32 not Size. Since shm_toc_lookup() reads the field without any lock, it has to be atomically readable, and we do not assume that for fields wider than 32 bits. Performance would be impossibly bad for entry counts approaching 2^32 anyway, so there is no need to try to preserve maximum width here. This is probably an academic issue, because even if reading int64 isn't atomic, the high order half would never change in practice. Still, it's a coding rule violation, so let's fix it. Adjust some other not-terribly-well-chosen data types too, and copy-edit some comments. Make shm_toc_attach's Asserts consistent with shm_toc_create's. None of this looks to be a live bug, so no need for back-patch. Discussion: https://postgr.es/m/16984.1496679541@sss.pgh.pa.us
59 lines
2.2 KiB
C
59 lines
2.2 KiB
C
/*-------------------------------------------------------------------------
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*
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* shm_toc.h
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* shared memory segment table of contents
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*
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* This is intended to provide a simple way to divide a chunk of shared
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* memory (probably dynamic shared memory allocated via dsm_create) into
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* a number of regions and keep track of the addresses of those regions or
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* key data structures within those regions. This is not intended to
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* scale to a large number of keys and will perform poorly if used that
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* way; if you need a large number of pointers, store them within some
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* other data structure within the segment and only put the pointer to
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* the data structure itself in the table of contents.
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*
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* Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/storage/shm_toc.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef SHM_TOC_H
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#define SHM_TOC_H
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#include "storage/shmem.h" /* for add_size() */
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/* shm_toc is an opaque type known only within shm_toc.c */
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typedef struct shm_toc shm_toc;
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extern shm_toc *shm_toc_create(uint64 magic, void *address, Size nbytes);
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extern shm_toc *shm_toc_attach(uint64 magic, void *address);
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extern void *shm_toc_allocate(shm_toc *toc, Size nbytes);
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extern Size shm_toc_freespace(shm_toc *toc);
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extern void shm_toc_insert(shm_toc *toc, uint64 key, void *address);
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extern void *shm_toc_lookup(shm_toc *toc, uint64 key, bool noError);
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/*
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* Tools for estimating how large a chunk of shared memory will be needed
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* to store a TOC and its dependent objects. Note: we don't really support
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* large numbers of keys, but it's convenient to declare number_of_keys
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* as a Size anyway.
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*/
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typedef struct
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{
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Size space_for_chunks;
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Size number_of_keys;
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} shm_toc_estimator;
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#define shm_toc_initialize_estimator(e) \
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((e)->space_for_chunks = 0, (e)->number_of_keys = 0)
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#define shm_toc_estimate_chunk(e, sz) \
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((e)->space_for_chunks = add_size((e)->space_for_chunks, BUFFERALIGN(sz)))
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#define shm_toc_estimate_keys(e, cnt) \
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((e)->number_of_keys = add_size((e)->number_of_keys, cnt))
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extern Size shm_toc_estimate(shm_toc_estimator *e);
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#endif /* SHM_TOC_H */
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