bind/lib/isc/rwlock.c

/*
 * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 *
 * SPDX-License-Identifier: MPL-2.0
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, you can obtain one at https://mozilla.org/MPL/2.0/.
 *
 * See the COPYRIGHT file distributed with this work for additional
 * information regarding copyright ownership.
 */

/*! \file */

#include <inttypes.h>
#include <stdbool.h>
#include <stddef.h>

#include <isc/atomic.h>
#include <isc/magic.h>
#include <isc/pause.h>
#include <isc/print.h>
#include <isc/rwlock.h>
#include <isc/util.h>

#define RWLOCK_MAGIC	  ISC_MAGIC('R', 'W', 'L', 'k')
#define VALID_RWLOCK(rwl) ISC_MAGIC_VALID(rwl, RWLOCK_MAGIC)

#ifndef RWLOCK_DEFAULT_READ_QUOTA
#define RWLOCK_DEFAULT_READ_QUOTA 4
#endif /* ifndef RWLOCK_DEFAULT_READ_QUOTA */

#ifndef RWLOCK_DEFAULT_WRITE_QUOTA
#define RWLOCK_DEFAULT_WRITE_QUOTA 4
#endif /* ifndef RWLOCK_DEFAULT_WRITE_QUOTA */

#ifndef RWLOCK_MAX_ADAPTIVE_COUNT
#define RWLOCK_MAX_ADAPTIVE_COUNT 100
#endif /* ifndef RWLOCK_MAX_ADAPTIVE_COUNT */

#ifdef ISC_RWLOCK_TRACE
#include <stdio.h> /* Required for fprintf/stderr. */

#include <isc/thread.h> /* Required for isc_thread_self(). */

static void
print_lock(const char *operation, isc__rwlock_t *rwl, isc_rwlocktype_t type) {
	fprintf(stderr,
		"rwlock %p thread %" PRIuPTR " %s(%s): "
		"write_requests=%u, write_completions=%u, "
		"cnt_and_flag=0x%x, readers_waiting=%u, "
		"write_granted=%u, write_quota=%u\n",
		rwl, isc_thread_self(), operation,
		(type == isc_rwlocktype_read ? "read" : "write"),
		atomic_load_acquire(&rwl->write_requests),
		atomic_load_acquire(&rwl->write_completions),
		atomic_load_acquire(&rwl->cnt_and_flag), rwl->readers_waiting,
		atomic_load_acquire(&rwl->write_granted), rwl->write_quota);
}
#endif /* ISC_RWLOCK_TRACE */

void
isc__rwlock_init(isc__rwlock_t *rwl, unsigned int read_quota,
		 unsigned int write_quota) {
	REQUIRE(rwl != NULL);

	/*
	 * In case there's trouble initializing, we zero magic now.  If all
	 * goes well, we'll set it to RWLOCK_MAGIC.
	 */
	rwl->magic = 0;

	atomic_init(&rwl->spins, 0);
	atomic_init(&rwl->write_requests, 0);
	atomic_init(&rwl->write_completions, 0);
	atomic_init(&rwl->cnt_and_flag, 0);
	rwl->readers_waiting = 0;
	atomic_init(&rwl->write_granted, 0);
	if (read_quota != 0) {
		UNEXPECTED_ERROR("read quota is not supported");
	}
	if (write_quota == 0) {
		write_quota = RWLOCK_DEFAULT_WRITE_QUOTA;
	}
	rwl->write_quota = write_quota;

	isc_mutex_init(&rwl->lock);

	isc_condition_init(&rwl->readable);
	isc_condition_init(&rwl->writeable);

	rwl->magic = RWLOCK_MAGIC;
}

void
isc__rwlock_destroy(isc__rwlock_t *rwl) {
	REQUIRE(VALID_RWLOCK(rwl));

	REQUIRE(atomic_load_acquire(&rwl->write_requests) ==
			atomic_load_acquire(&rwl->write_completions) &&
		atomic_load_acquire(&rwl->cnt_and_flag) == 0 &&
		rwl->readers_waiting == 0);

	rwl->magic = 0;
	isc_condition_destroy(&rwl->readable);
	isc_condition_destroy(&rwl->writeable);
	isc_mutex_destroy(&rwl->lock);
}

/*
 * When some architecture-dependent atomic operations are available,
 * rwlock can be more efficient than the generic algorithm defined below.
 * The basic algorithm is described in the following URL:
 *   http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html
 *
 * The key is to use the following integer variables modified atomically:
 *   write_requests, write_completions, and cnt_and_flag.
 *
 * write_requests and write_completions act as a waiting queue for writers
 * in order to ensure the FIFO order.  Both variables begin with the initial
 * value of 0.  When a new writer tries to get a write lock, it increments
 * write_requests and gets the previous value of the variable as a "ticket".
 * When write_completions reaches the ticket number, the new writer can start
 * writing.  When the writer completes its work, it increments
 * write_completions so that another new writer can start working.  If the
 * write_requests is not equal to write_completions, it means a writer is now
 * working or waiting.  In this case, a new readers cannot start reading, or
 * in other words, this algorithm basically prefers writers.
 *
 * cnt_and_flag is a "lock" shared by all readers and writers.  This integer
 * variable is a kind of structure with two members: writer_flag (1 bit) and
 * reader_count (31 bits).  The writer_flag shows whether a writer is working,
 * and the reader_count shows the number of readers currently working or almost
 * ready for working.  A writer who has the current "ticket" tries to get the
 * lock by exclusively setting the writer_flag to 1, provided that the whole
 * 32-bit is 0 (meaning no readers or writers working).  On the other hand,
 * a new reader tries to increment the "reader_count" field provided that
 * the writer_flag is 0 (meaning there is no writer working).
 *
 * If some of the above operations fail, the reader or the writer sleeps
 * until the related condition changes.  When a working reader or writer
 * completes its work, some readers or writers are sleeping, and the condition
 * that suspended the reader or writer has changed, it wakes up the sleeping
 * readers or writers.
 *
 * As already noted, this algorithm basically prefers writers.  In order to
 * prevent readers from starving, however, the algorithm also introduces the
 * "writer quota" (Q).  When Q consecutive writers have completed their work,
 * suspending readers, the last writer will wake up the readers, even if a new
 * writer is waiting.
 *
 * Implementation specific note: due to the combination of atomic operations
 * and a mutex lock, ordering between the atomic operation and locks can be
 * very sensitive in some cases.  In particular, it is generally very important
 * to check the atomic variable that requires a reader or writer to sleep after
 * locking the mutex and before actually sleeping; otherwise, it could be very
 * likely to cause a deadlock.  For example, assume "var" is a variable
 * atomically modified, then the corresponding code would be:
 *	if (var == need_sleep) {
 *		LOCK(lock);
 *		if (var == need_sleep)
 *			WAIT(cond, lock);
 *		UNLOCK(lock);
 *	}
 * The second check is important, since "var" is protected by the atomic
 * operation, not by the mutex, and can be changed just before sleeping.
 * (The first "if" could be omitted, but this is also important in order to
 * make the code efficient by avoiding the use of the mutex unless it is
 * really necessary.)
 */

#define WRITER_ACTIVE 0x1
#define READER_INCR   0x2

static void
rwlock_lock(isc__rwlock_t *rwl, isc_rwlocktype_t type) {
	int32_t cntflag;

	REQUIRE(VALID_RWLOCK(rwl));

#ifdef ISC_RWLOCK_TRACE
	print_lock("prelock", rwl, type);
#endif /* ifdef ISC_RWLOCK_TRACE */

	if (type == isc_rwlocktype_read) {
		if (atomic_load_acquire(&rwl->write_requests) !=
		    atomic_load_acquire(&rwl->write_completions))
		{
			/* there is a waiting or active writer */
			LOCK(&rwl->lock);
			if (atomic_load_acquire(&rwl->write_requests) !=
			    atomic_load_acquire(&rwl->write_completions))
			{
				rwl->readers_waiting++;
				WAIT(&rwl->readable, &rwl->lock);
				rwl->readers_waiting--;
			}
			UNLOCK(&rwl->lock);
		}

		cntflag = atomic_fetch_add_release(&rwl->cnt_and_flag,
						   READER_INCR);
		POST(cntflag);
		while (1) {
			if ((atomic_load_acquire(&rwl->cnt_and_flag) &
			     WRITER_ACTIVE) == 0)
			{
				break;
			}

			/* A writer is still working */
			LOCK(&rwl->lock);
			rwl->readers_waiting++;
			if ((atomic_load_acquire(&rwl->cnt_and_flag) &
			     WRITER_ACTIVE) != 0)
			{
				WAIT(&rwl->readable, &rwl->lock);
			}
			rwl->readers_waiting--;
			UNLOCK(&rwl->lock);

			/*
			 * Typically, the reader should be able to get a lock
			 * at this stage:
			 *   (1) there should have been no pending writer when
			 *       the reader was trying to increment the
			 *       counter; otherwise, the writer should be in
			 *       the waiting queue, preventing the reader from
			 *       proceeding to this point.
			 *   (2) once the reader increments the counter, no
			 *       more writer can get a lock.
			 * Still, it is possible another writer can work at
			 * this point, e.g. in the following scenario:
			 *   A previous writer unlocks the writer lock.
			 *   This reader proceeds to point (1).
			 *   A new writer appears, and gets a new lock before
			 *   the reader increments the counter.
			 *   The reader then increments the counter.
			 *   The previous writer notices there is a waiting
			 *   reader who is almost ready, and wakes it up.
			 * So, the reader needs to confirm whether it can now
			 * read explicitly (thus we loop).  Note that this is
			 * not an infinite process, since the reader has
			 * incremented the counter at this point.
			 */
		}

		/*
		 * If we are temporarily preferred to writers due to the writer
		 * quota, reset the condition (race among readers doesn't
		 * matter).
		 */
		atomic_store_release(&rwl->write_granted, 0);
	} else {
		int32_t prev_writer;

		/* enter the waiting queue, and wait for our turn */
		prev_writer = atomic_fetch_add_release(&rwl->write_requests, 1);
		while (atomic_load_acquire(&rwl->write_completions) !=
		       prev_writer)
		{
			LOCK(&rwl->lock);
			if (atomic_load_acquire(&rwl->write_completions) !=
			    prev_writer)
			{
				WAIT(&rwl->writeable, &rwl->lock);
				UNLOCK(&rwl->lock);
				continue;
			}
			UNLOCK(&rwl->lock);
			break;
		}

		while (!atomic_compare_exchange_weak_acq_rel(
			&rwl->cnt_and_flag, &(int_fast32_t){ 0 },
			WRITER_ACTIVE))
		{
			/* Another active reader or writer is working. */
			LOCK(&rwl->lock);
			if (atomic_load_acquire(&rwl->cnt_and_flag) != 0) {
				WAIT(&rwl->writeable, &rwl->lock);
			}
			UNLOCK(&rwl->lock);
		}

		INSIST((atomic_load_acquire(&rwl->cnt_and_flag) &
			WRITER_ACTIVE));
		atomic_fetch_add_release(&rwl->write_granted, 1);
	}

#ifdef ISC_RWLOCK_TRACE
	print_lock("postlock", rwl, type);
#endif /* ifdef ISC_RWLOCK_TRACE */
}

void
isc__rwlock_lock(isc__rwlock_t *rwl, isc_rwlocktype_t type) {
	int32_t cnt = 0;
	int32_t spins = atomic_load_acquire(&rwl->spins) * 2 + 10;
	int32_t max_cnt = ISC_MAX(spins, RWLOCK_MAX_ADAPTIVE_COUNT);

	do {
		if (cnt++ >= max_cnt) {
			rwlock_lock(rwl, type);
			break;
		}
		isc_pause();
	} while (isc_rwlock_trylock(rwl, type) != ISC_R_SUCCESS);

	atomic_fetch_add_release(&rwl->spins, (cnt - spins) / 8);
}

isc_result_t
isc__rwlock_trylock(isc__rwlock_t *rwl, isc_rwlocktype_t type) {
	int32_t cntflag;

	REQUIRE(VALID_RWLOCK(rwl));

#ifdef ISC_RWLOCK_TRACE
	print_lock("prelock", rwl, type);
#endif /* ifdef ISC_RWLOCK_TRACE */

	if (type == isc_rwlocktype_read) {
		/* If a writer is waiting or working, we fail. */
		if (atomic_load_acquire(&rwl->write_requests) !=
		    atomic_load_acquire(&rwl->write_completions))
		{
			return (ISC_R_LOCKBUSY);
		}

		/* Otherwise, be ready for reading. */
		cntflag = atomic_fetch_add_release(&rwl->cnt_and_flag,
						   READER_INCR);
		if ((cntflag & WRITER_ACTIVE) != 0) {
			/*
			 * A writer is working.  We lose, and cancel the read
			 * request.
			 */
			cntflag = atomic_fetch_sub_release(&rwl->cnt_and_flag,
							   READER_INCR);
			/*
			 * If no other readers are waiting and we've suspended
			 * new writers in this short period, wake them up.
			 */
			if (cntflag == READER_INCR &&
			    atomic_load_acquire(&rwl->write_completions) !=
				    atomic_load_acquire(&rwl->write_requests))
			{
				LOCK(&rwl->lock);
				BROADCAST(&rwl->writeable);
				UNLOCK(&rwl->lock);
			}

			return (ISC_R_LOCKBUSY);
		}
	} else {
		/* Try locking without entering the waiting queue. */
		int_fast32_t zero = 0;
		if (!atomic_compare_exchange_strong_acq_rel(
			    &rwl->cnt_and_flag, &zero, WRITER_ACTIVE))
		{
			return (ISC_R_LOCKBUSY);
		}

		/*
		 * XXXJT: jump into the queue, possibly breaking the writer
		 * order.
		 */
		atomic_fetch_sub_release(&rwl->write_completions, 1);
		atomic_fetch_add_release(&rwl->write_granted, 1);
	}

#ifdef ISC_RWLOCK_TRACE
	print_lock("postlock", rwl, type);
#endif /* ifdef ISC_RWLOCK_TRACE */

	return (ISC_R_SUCCESS);
}

isc_result_t
isc__rwlock_tryupgrade(isc__rwlock_t *rwl) {
	REQUIRE(VALID_RWLOCK(rwl));

	int_fast32_t reader_incr = READER_INCR;

	/* Try to acquire write access. */
	atomic_compare_exchange_strong_acq_rel(&rwl->cnt_and_flag, &reader_incr,
					       WRITER_ACTIVE);
	/*
	 * There must have been no writer, and there must have
	 * been at least one reader.
	 */
	INSIST((reader_incr & WRITER_ACTIVE) == 0 &&
	       (reader_incr & ~WRITER_ACTIVE) != 0);

	if (reader_incr == READER_INCR) {
		/*
		 * We are the only reader and have been upgraded.
		 * Now jump into the head of the writer waiting queue.
		 */
		atomic_fetch_sub_release(&rwl->write_completions, 1);
	} else {
		return (ISC_R_LOCKBUSY);
	}

	return (ISC_R_SUCCESS);
}

void
isc__rwlock_unlock(isc__rwlock_t *rwl, isc_rwlocktype_t type) {
	int32_t prev_cnt;

	REQUIRE(VALID_RWLOCK(rwl));

#ifdef ISC_RWLOCK_TRACE
	print_lock("preunlock", rwl, type);
#endif /* ifdef ISC_RWLOCK_TRACE */

	if (type == isc_rwlocktype_read) {
		prev_cnt = atomic_fetch_sub_release(&rwl->cnt_and_flag,
						    READER_INCR);
		/*
		 * If we're the last reader and any writers are waiting, wake
		 * them up.  We need to wake up all of them to ensure the
		 * FIFO order.
		 */
		if (prev_cnt == READER_INCR &&
		    atomic_load_acquire(&rwl->write_completions) !=
			    atomic_load_acquire(&rwl->write_requests))
		{
			LOCK(&rwl->lock);
			BROADCAST(&rwl->writeable);
			UNLOCK(&rwl->lock);
		}
	} else {
		bool wakeup_writers = true;

		/*
		 * Reset the flag, and (implicitly) tell other writers
		 * we are done.
		 */
		atomic_fetch_sub_release(&rwl->cnt_and_flag, WRITER_ACTIVE);
		atomic_fetch_add_release(&rwl->write_completions, 1);

		if ((atomic_load_acquire(&rwl->write_granted) >=
		     rwl->write_quota) ||
		    (atomic_load_acquire(&rwl->write_requests) ==
		     atomic_load_acquire(&rwl->write_completions)) ||
		    (atomic_load_acquire(&rwl->cnt_and_flag) & ~WRITER_ACTIVE))
		{
			/*
			 * We have passed the write quota, no writer is
			 * waiting, or some readers are almost ready, pending
			 * possible writers.  Note that the last case can
			 * happen even if write_requests != write_completions
			 * (which means a new writer in the queue), so we need
			 * to catch the case explicitly.
			 */
			LOCK(&rwl->lock);
			if (rwl->readers_waiting > 0) {
				wakeup_writers = false;
				BROADCAST(&rwl->readable);
			}
			UNLOCK(&rwl->lock);
		}

		if ((atomic_load_acquire(&rwl->write_requests) !=
		     atomic_load_acquire(&rwl->write_completions)) &&
		    wakeup_writers)
		{
			LOCK(&rwl->lock);
			BROADCAST(&rwl->writeable);
			UNLOCK(&rwl->lock);
		}
	}

#ifdef ISC_RWLOCK_TRACE
	print_lock("postunlock", rwl, type);
#endif /* ifdef ISC_RWLOCK_TRACE */
}