sync0sharded_rw.h

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/**************************************************/ /**
 @file include/sync0sharded_rw.h
 
 The sharded read-write lock (for threads).
 
 The s-lock scales better than in single rw-lock,
 but the x-lock is much slower.
 
 *******************************************************/
 
#ifndef sync0sharded_rw_h
#define sync0sharded_rw_h
 
#include "sync0rw.h"
#include "ut0cpu_cache.h"
#include "ut0rnd.h"
#include "ut0ut.h"
 
#ifndef UNIV_HOTBACKUP
#ifndef UNIV_LIBRARY
 
/** Rw-lock with very fast, highly concurrent s-lock but slower x-lock.
It's basically array of rw-locks. When s-lock is being acquired, single
rw-lock from array is selected randomly and s-locked. Therefore, all
rw-locks from array has to be x-locked when x-lock is being acquired.
 
Purpose of this data structure is to reduce contention on single atomic
in single rw-lock when a lot of threads need to acquire s-lock very often,
but x-lock is very rare. */
class Sharded_rw_lock {
 public:
  void create(
#ifdef UNIV_PFS_RWLOCK
      mysql_pfs_key_t pfs_key,
#endif
      latch_id_t latch_id, size_t n_shards) {
    ut_ad(ut_is_2pow(n_shards));
    m_n_shards = n_shards;
 
    m_shards = static_cast<Shard *>(
        ut::zalloc_withkey(UT_NEW_THIS_FILE_PSI_KEY, sizeof(Shard) * n_shards));
 
    for_each([
#ifdef UNIV_PFS_RWLOCK
                 pfs_key,
#endif
                 latch_id](rw_lock_t &lock) {
      static_cast<void>(latch_id);  // clang -Wunused-lambda-capture
      rw_lock_create(pfs_key, &lock, latch_id);
    });
  }
 
  void free() {
    ut_a(m_shards != nullptr);
 
    for_each([](rw_lock_t &lock) { rw_lock_free(&lock); });
 
    ut::free(m_shards);
    m_shards = nullptr;
    m_n_shards = 0;
  }
 
  size_t s_lock(ut::Location location) {
    const size_t shard_no =
        default_indexer_t<>::get_rnd_index() & (m_n_shards - 1);
    rw_lock_s_lock_gen(&m_shards[shard_no], 0, location);
    return shard_no;
  }
 
  void s_unlock(size_t shard_no) {
    ut_a(shard_no < m_n_shards);
    rw_lock_s_unlock(&m_shards[shard_no]);
  }
  /** Checks if there is a thread requesting an x-latch waiting for threads to
  release their s-latches on given shard.
  @param[in]  shard_no  The shard to check.
  @return true iff there is an x-latcher blocked by s-latchers on shard_no. */
  bool is_x_blocked_by_s(size_t shard_no) {
    ut_a(shard_no < m_n_shards);
    return m_shards[shard_no].is_x_blocked_by_s();
  }
  /**
  Tries to obtain exclusive latch - similar to x_lock(), but non-blocking, and
  thus can fail.
  @return true iff succeeded to acquire the exclusive latch
  */
  bool try_x_lock(ut::Location location) {
    for (size_t shard_no = 0; shard_no < m_n_shards; ++shard_no) {
      if (!rw_lock_x_lock_nowait(&m_shards[shard_no], location)) {
        while (0 < shard_no--) {
          rw_lock_x_unlock(&m_shards[shard_no]);
        }
        return (false);
      }
    }
    return (true);
  }
 
  void x_lock(ut::Location location) {
    for_each([location](rw_lock_t &lock) {
      rw_lock_x_lock_gen(&lock, 0, location);
    });
  }
 
  void x_unlock() {
    for_each([](rw_lock_t &lock) { rw_lock_x_unlock(&lock); });
  }
 
#ifdef UNIV_DEBUG
  bool s_own(size_t shard_no) const {
    return rw_lock_own(&m_shards[shard_no], RW_LOCK_S);
  }
 
  bool x_own() const { return rw_lock_own(&m_shards[0], RW_LOCK_X); }
#endif /* !UNIV_DEBUG */
 
 private:
  using Shard = ut::Cacheline_padded<rw_lock_t>;
 
  template <typename F>
  void for_each(F f) {
    std::for_each(m_shards, m_shards + m_n_shards, f);
  }
 
  Shard *m_shards = nullptr;
 
  size_t m_n_shards = 0;
};
 
#else /* !UNIV_LIBRARY */
 
/* For UNIV_LIBRARY, rw_lock is no-op, so sharded rw-lock is also no-op. */
 
class Sharded_rw_lock {
 public:
  void create(
#ifdef UNIV_PFS_RWLOCK
      mysql_pfs_key_t pfs_key,
#endif
      latch_id_t latch_id, size_t n_shards) {
  }
 
  void free() {}
 
  size_t s_lock() { return 0; }
 
  void s_unlock(size_t shard_no) { ut_a(shard_no == 0); }
 
  void x_lock() {}
 
  void x_unlock() {}
};
 
#endif /* UNIV_LIBRARY */
#endif /* UNIV_HOTBACKUP */
 
#endif /* sync0sharded_rw.h */