162306a36Sopenharmony_ci/* SPDX-License-Identifier: GPL-2.0 */
262306a36Sopenharmony_ci#ifndef PERF_SHARDED_MUTEX_H
362306a36Sopenharmony_ci#define PERF_SHARDED_MUTEX_H
462306a36Sopenharmony_ci
562306a36Sopenharmony_ci#include "mutex.h"
662306a36Sopenharmony_ci#include "hashmap.h"
762306a36Sopenharmony_ci
862306a36Sopenharmony_ci/*
962306a36Sopenharmony_ci * In a situation where a lock is needed per object, having a mutex can be
1062306a36Sopenharmony_ci * relatively memory expensive (40 bytes on x86-64). If the object can be
1162306a36Sopenharmony_ci * constantly hashed, a sharded mutex is an alternative global pool of mutexes
1262306a36Sopenharmony_ci * where the mutex is looked up from a hash value. This can lead to collisions
1362306a36Sopenharmony_ci * if the number of shards isn't large enough.
1462306a36Sopenharmony_ci */
1562306a36Sopenharmony_cistruct sharded_mutex {
1662306a36Sopenharmony_ci	/* mutexes array is 1<<cap_bits in size. */
1762306a36Sopenharmony_ci	unsigned int cap_bits;
1862306a36Sopenharmony_ci	struct mutex mutexes[];
1962306a36Sopenharmony_ci};
2062306a36Sopenharmony_ci
2162306a36Sopenharmony_cistruct sharded_mutex *sharded_mutex__new(size_t num_shards);
2262306a36Sopenharmony_civoid sharded_mutex__delete(struct sharded_mutex *sm);
2362306a36Sopenharmony_ci
2462306a36Sopenharmony_cistatic inline struct mutex *sharded_mutex__get_mutex(struct sharded_mutex *sm, size_t hash)
2562306a36Sopenharmony_ci{
2662306a36Sopenharmony_ci	return &sm->mutexes[hash_bits(hash, sm->cap_bits)];
2762306a36Sopenharmony_ci}
2862306a36Sopenharmony_ci
2962306a36Sopenharmony_ci#endif  /* PERF_SHARDED_MUTEX_H */
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