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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 07:24:22 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 07:24:22 +0000 |
commit | 45d6379135504814ab723b57f0eb8be23393a51d (patch) | |
tree | d4f2ec4acca824a8446387a758b0ce4238a4dffa /lib/isc/rwlock.c | |
parent | Initial commit. (diff) | |
download | bind9-upstream.tar.xz bind9-upstream.zip |
Adding upstream version 1:9.16.44.upstream/1%9.16.44upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | lib/isc/rwlock.c | 646 |
1 files changed, 646 insertions, 0 deletions
diff --git a/lib/isc/rwlock.c b/lib/isc/rwlock.c new file mode 100644 index 0000000..c69b6d7 --- /dev/null +++ b/lib/isc/rwlock.c @@ -0,0 +1,646 @@ +/* + * 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> + +#if defined(sun) && (defined(__sparc) || defined(__sparc__)) +#include <synch.h> /* for smt_pause(3c) */ +#endif /* if defined(sun) && (defined(__sparc) || defined(__sparc__)) */ + +#include <isc/atomic.h> +#include <isc/magic.h> +#include <isc/platform.h> +#include <isc/print.h> +#include <isc/rwlock.h> +#include <isc/util.h> + +#if USE_PTHREAD_RWLOCK + +#include <errno.h> +#include <pthread.h> + +void +isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota, + unsigned int write_quota) { + UNUSED(read_quota); + UNUSED(write_quota); + REQUIRE(pthread_rwlock_init(&rwl->rwlock, NULL) == 0); + atomic_init(&rwl->downgrade, false); +} + +isc_result_t +isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) { + switch (type) { + case isc_rwlocktype_read: + REQUIRE(pthread_rwlock_rdlock(&rwl->rwlock) == 0); + break; + case isc_rwlocktype_write: + while (true) { + REQUIRE(pthread_rwlock_wrlock(&rwl->rwlock) == 0); + /* Unlock if in middle of downgrade operation */ + if (atomic_load_acquire(&rwl->downgrade)) { + REQUIRE(pthread_rwlock_unlock(&rwl->rwlock) == + 0); + while (atomic_load_acquire(&rwl->downgrade)) { + } + continue; + } + break; + } + break; + default: + UNREACHABLE(); + } + return (ISC_R_SUCCESS); +} + +isc_result_t +isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) { + int ret = 0; + switch (type) { + case isc_rwlocktype_read: + ret = pthread_rwlock_tryrdlock(&rwl->rwlock); + break; + case isc_rwlocktype_write: + ret = pthread_rwlock_trywrlock(&rwl->rwlock); + if ((ret == 0) && atomic_load_acquire(&rwl->downgrade)) { + isc_rwlock_unlock(rwl, type); + return (ISC_R_LOCKBUSY); + } + break; + default: + UNREACHABLE(); + } + + switch (ret) { + case 0: + return (ISC_R_SUCCESS); + case EBUSY: + return (ISC_R_LOCKBUSY); + case EAGAIN: + return (ISC_R_LOCKBUSY); + default: + UNREACHABLE(); + } +} + +isc_result_t +isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) { + UNUSED(type); + REQUIRE(pthread_rwlock_unlock(&rwl->rwlock) == 0); + return (ISC_R_SUCCESS); +} + +isc_result_t +isc_rwlock_tryupgrade(isc_rwlock_t *rwl) { + UNUSED(rwl); + return (ISC_R_LOCKBUSY); +} + +void +isc_rwlock_downgrade(isc_rwlock_t *rwl) { + isc_result_t result; + atomic_store_release(&rwl->downgrade, true); + result = isc_rwlock_unlock(rwl, isc_rwlocktype_write); + RUNTIME_CHECK(result == ISC_R_SUCCESS); + result = isc_rwlock_lock(rwl, isc_rwlocktype_read); + RUNTIME_CHECK(result == ISC_R_SUCCESS); + atomic_store_release(&rwl->downgrade, false); +} + +void +isc_rwlock_destroy(isc_rwlock_t *rwl) { + pthread_rwlock_destroy(&rwl->rwlock); +} + +#else /* if USE_PTHREAD_RWLOCK */ + +#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 */ + +#if defined(_MSC_VER) +#include <intrin.h> +#define isc_rwlock_pause() YieldProcessor() +#elif defined(__x86_64__) +#include <immintrin.h> +#define isc_rwlock_pause() _mm_pause() +#elif defined(__i386__) +#define isc_rwlock_pause() __asm__ __volatile__("rep; nop") +#elif defined(__ia64__) +#define isc_rwlock_pause() __asm__ __volatile__("hint @pause") +#elif defined(__arm__) && HAVE_ARM_YIELD +#define isc_rwlock_pause() __asm__ __volatile__("yield") +#elif defined(sun) && (defined(__sparc) || defined(__sparc__)) +#define isc_rwlock_pause() smt_pause() +#elif (defined(__sparc) || defined(__sparc__)) && HAVE_SPARC_PAUSE +#define isc_rwlock_pause() __asm__ __volatile__("pause") +#elif defined(__ppc__) || defined(_ARCH_PPC) || defined(_ARCH_PWR) || \ + defined(_ARCH_PWR2) || defined(_POWER) +#define isc_rwlock_pause() __asm__ volatile("or 27,27,27") +#else /* if defined(_MSC_VER) */ +#define isc_rwlock_pause() +#endif /* if defined(_MSC_VER) */ + +static isc_result_t +isc__rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type); + +#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(__FILE__, __LINE__, + "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; + (void)isc_condition_destroy(&rwl->readable); + (void)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 isc_result_t +isc__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 */ + + return (ISC_R_SUCCESS); +} + +isc_result_t +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); + isc_result_t result = ISC_R_SUCCESS; + + do { + if (cnt++ >= max_cnt) { + result = isc__rwlock_lock(rwl, type); + break; + } + isc_rwlock_pause(); + } while (isc_rwlock_trylock(rwl, type) != ISC_R_SUCCESS); + + atomic_fetch_add_release(&rwl->spins, (cnt - spins) / 8); + + return (result); +} + +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_downgrade(isc_rwlock_t *rwl) { + int32_t prev_readers; + + REQUIRE(VALID_RWLOCK(rwl)); + + /* Become an active reader. */ + prev_readers = atomic_fetch_add_release(&rwl->cnt_and_flag, + READER_INCR); + /* We must have been a writer. */ + INSIST((prev_readers & WRITER_ACTIVE) != 0); + + /* Complete write */ + atomic_fetch_sub_release(&rwl->cnt_and_flag, WRITER_ACTIVE); + atomic_fetch_add_release(&rwl->write_completions, 1); + + /* Resume other readers */ + LOCK(&rwl->lock); + if (rwl->readers_waiting > 0) { + BROADCAST(&rwl->readable); + } + UNLOCK(&rwl->lock); +} + +isc_result_t +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 */ + + return (ISC_R_SUCCESS); +} + +#endif /* USE_PTHREAD_RWLOCK */ |