116 lines
4.2 KiB
C
116 lines
4.2 KiB
C
/* Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License, version 2.0,
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as published by the Free Software Foundation.
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This program is also distributed with certain software (including
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but not limited to OpenSSL) that is licensed under separate terms,
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as designated in a particular file or component or in included license
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documentation. The authors of MySQL hereby grant you an additional
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permission to link the program and your derivative works with the
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separately licensed software that they have included with MySQL.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License, version 2.0, for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
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#ifndef COMPONENTS_SERVICES_THR_RWLOCK_BITS_H
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#define COMPONENTS_SERVICES_THR_RWLOCK_BITS_H
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/**
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@file
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MySQL rwlock ABI.
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There are two "layers":
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1) native_rw_*()
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Functions that map directly down to OS primitives.
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Windows - SRWLock
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Other OSes - pthread
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2) mysql_rw*()
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Functions that include Performance Schema instrumentation.
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See include/mysql/psi/mysql_thread.h
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This file also includes rw_pr_*(), which implements a special
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version of rwlocks that prefer readers. The P_S version of these
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are mysql_prlock_*() - see include/mysql/psi/mysql_thread.h
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*/
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#include <stddef.h>
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#include <sys/types.h>
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#ifdef _WIN32
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#include <windows.h>
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#endif
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#include <mysql/components/services/my_thread_bits.h>
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#include <mysql/components/services/thr_cond_bits.h>
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#include <mysql/components/services/thr_mutex_bits.h>
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#ifdef _WIN32
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struct native_rw_lock_t {
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SRWLOCK srwlock; /* native reader writer lock */
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BOOL have_exclusive_srwlock; /* used for unlock */
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};
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#else
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typedef pthread_rwlock_t native_rw_lock_t;
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#endif
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/**
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Portable implementation of special type of read-write locks.
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These locks have two properties which are unusual for rwlocks:
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1) They "prefer readers" in the sense that they do not allow
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situations in which rwlock is rd-locked and there is a
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pending rd-lock which is blocked (e.g. due to pending
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request for wr-lock).
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This is a stronger guarantee than one which is provided for
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PTHREAD_RWLOCK_PREFER_READER_NP rwlocks in Linux.
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MDL subsystem deadlock detector relies on this property for
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its correctness.
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2) They are optimized for uncontended wr-lock/unlock case.
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This is scenario in which they are most oftenly used
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within MDL subsystem. Optimizing for it gives significant
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performance improvements in some of tests involving many
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connections.
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Another important requirement imposed on this type of rwlock
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by the MDL subsystem is that it should be OK to destroy rwlock
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object which is in unlocked state even though some threads might
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have not yet fully left unlock operation for it (of course there
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is an external guarantee that no thread will try to lock rwlock
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which is destroyed).
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Putting it another way the unlock operation should not access
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rwlock data after changing its state to unlocked.
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TODO/FIXME: We should consider alleviating this requirement as
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it blocks us from doing certain performance optimizations.
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*/
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struct rw_pr_lock_t {
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/**
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Lock which protects the structure.
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Also held for the duration of wr-lock.
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*/
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native_mutex_t lock;
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/**
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Condition variable which is used to wake-up
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writers waiting for readers to go away.
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*/
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native_cond_t no_active_readers;
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/** Number of active readers. */
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unsigned int active_readers;
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/** Number of writers waiting for readers to go away. */
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unsigned int writers_waiting_readers;
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/** Indicates whether there is an active writer. */
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bool active_writer;
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/** Thread holding wr-lock (for debug purposes only). */
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my_thread_t writer_thread;
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};
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#endif /* COMPONENTS_SERVICES_THR_RWLOCK_BITS_H */
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