[libc-commits] [libc] [libc] add rwlock (PR #94156)
via libc-commits
libc-commits at lists.llvm.org
Sun Jun 2 12:01:58 PDT 2024
================
@@ -0,0 +1,518 @@
+//===--- Implementation of a Linux RwLock class ---------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_LIBC_SRC_SUPPORT_THREADS_LINUX_RWLOCK_H
+#define LLVM_LIBC_SRC_SUPPORT_THREADS_LINUX_RWLOCK_H
+
+#include "hdr/errno_macros.h"
+#include "hdr/types/pid_t.h"
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/OSUtil/syscall.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_assert.h"
+#include "src/__support/threads/linux/futex_utils.h"
+#include "src/__support/threads/linux/futex_word.h"
+#include "src/__support/threads/linux/raw_mutex.h"
+#include "src/__support/threads/sleep.h"
+
+#ifndef LIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT
+#define LIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT 100
+#endif
+
+#ifndef LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
+#define LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY 1
+#warning "LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY is not defined, defaulting to 1"
+#endif
+
+#if LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
+#include "src/__support/time/linux/monotonicity.h"
+#endif
+
+namespace LIBC_NAMESPACE {
+class RwLock {
+private:
+ class WaitingQueue final : private RawMutex {
+ FutexWordType pending_reader;
+ FutexWordType pending_writer;
+ Futex reader_serialization;
+ Futex writer_serialization;
+
+ public:
+ class Guard {
+ WaitingQueue &queue;
+
+ LIBC_INLINE constexpr Guard(WaitingQueue &queue) : queue(queue) {}
+
+ public:
+ LIBC_INLINE ~Guard() { queue.unlock(); }
+ LIBC_INLINE FutexWordType &pending_reader() {
+ return queue.pending_reader;
+ }
+ LIBC_INLINE FutexWordType &pending_writer() {
+ return queue.pending_writer;
+ }
+ LIBC_INLINE FutexWordType &reader_serialization() {
+ return queue.reader_serialization.val;
+ }
+ LIBC_INLINE FutexWordType &writer_serialization() {
+ return queue.writer_serialization.val;
+ }
+ friend RwLock;
+ };
+
+ public:
+ LIBC_INLINE constexpr WaitingQueue()
+ : RawMutex(), pending_reader(0), pending_writer(0),
+ reader_serialization(0), writer_serialization(0) {}
+ LIBC_INLINE Guard acquire() {
+ this->lock();
+ return Guard(*this);
+ }
+ LIBC_INLINE long reader_wait(FutexWordType expected,
+ cpp::optional<Futex::Timeout> timeout,
+ bool is_pshared) {
+ return reader_serialization.wait(expected, timeout, is_pshared);
+ }
+ LIBC_INLINE long reader_notify_all(bool is_pshared) {
+ return reader_serialization.notify_all(is_pshared);
+ }
+ LIBC_INLINE long writer_wait(FutexWordType expected,
+ cpp::optional<Futex::Timeout> timeout,
+ bool is_pshared) {
+ return writer_serialization.wait(expected, timeout, is_pshared);
+ }
+ LIBC_INLINE long writer_notify_one(bool is_pshared) {
+ return writer_serialization.notify_one(is_pshared);
+ }
+ };
+
+public:
+ enum class Preference : char { Reader, Writer };
+ enum class LockResult {
+ Success = 0,
+ TimedOut = ETIMEDOUT,
+ Overflow = EAGAIN,
+ Busy = EBUSY,
+ Deadlock = EDEADLOCK,
+ PermissionDenied = EPERM,
+ };
+
+private:
+ // The State of the RwLock is stored in a 32-bit word, consisting of the
+ // following components:
+ // -----------------------------------------------
+ // | Range | Description |
+ // ===============================================
+ // | 0 | Pending Reader Bit |
+ // -----------------------------------------------
+ // | 1 | Pending Writer Bit |
+ // -----------------------------------------------
+ // | 2-30 | Active Reader Count |
+ // -----------------------------------------------
+ // | 31 | Active Writer Bit |
+ // -----------------------------------------------
+ class State {
+ // We use the signed interger as the state type. It is easier
+ // to handle state trasitions and detections using signed integers.
+ using Type = int32_t;
+
+ // Shift amounts to access the components of the state.
+ LIBC_INLINE_VAR static constexpr Type PENDING_READER_SHIFT = 0;
+ LIBC_INLINE_VAR static constexpr Type PENDING_WRITER_SHIFT = 1;
+ LIBC_INLINE_VAR static constexpr Type ACTIVE_READER_SHIFT = 2;
+ LIBC_INLINE_VAR static constexpr Type ACTIVE_WRITER_SHIFT = 31;
+
+ // Bitmasks to access the components of the state.
+ LIBC_INLINE_VAR static constexpr Type PENDING_READER_BIT =
+ 1 << PENDING_READER_SHIFT;
+ LIBC_INLINE_VAR static constexpr Type PENDING_WRITER_BIT =
+ 1 << PENDING_WRITER_SHIFT;
+ LIBC_INLINE_VAR static constexpr Type ACTIVE_READER_COUNT_UNIT =
+ 1 << ACTIVE_READER_SHIFT;
+ LIBC_INLINE_VAR static constexpr Type ACTIVE_WRITER_BIT =
+ 1 << ACTIVE_WRITER_SHIFT;
+ LIBC_INLINE_VAR static constexpr Type PENDING_MASK =
+ PENDING_READER_BIT | PENDING_WRITER_BIT;
+
+ private:
+ Type state;
+
+ public:
+ // Construction and conversion functions.
+ LIBC_INLINE constexpr State(Type state = 0) : state(state) {}
+ LIBC_INLINE constexpr operator Type() const { return state; }
+
+ // Utilities to check the state of the RwLock.
+ LIBC_INLINE constexpr bool has_active_writer() const { return state < 0; }
+ LIBC_INLINE constexpr bool has_active_reader() const {
+ return state > ACTIVE_READER_COUNT_UNIT;
+ }
+ LIBC_INLINE constexpr bool has_acitve_owner() const {
+ return has_active_reader() || has_active_writer();
+ }
+ LIBC_INLINE constexpr bool has_last_reader() const {
+ return (state >> ACTIVE_READER_SHIFT) == 1;
+ }
+ LIBC_INLINE constexpr bool has_pending_writer() const {
+ return state & PENDING_WRITER_BIT;
+ }
+ LIBC_INLINE constexpr bool has_pending() const {
+ return state & PENDING_MASK;
+ }
+ LIBC_INLINE constexpr State set_writer_bit() const {
+ return State(state | ACTIVE_WRITER_BIT);
+ }
+ // The preference parameter changes the behavior of the lock acquisition
+ // if there are both readers and writers waiting for the lock. If writers
+ // are preferred, reader acquisition will be blocked until all pending
+ // writers are served.
+ LIBC_INLINE bool can_acquire_reader(Preference preference) const {
+ switch (preference) {
+ case Preference::Reader:
+ return !has_active_writer();
+ case Preference::Writer:
+ return !has_active_writer() && !has_pending_writer();
+ }
+ }
+ LIBC_INLINE bool can_acquire_writer(Preference /*unused*/) const {
+ return !has_acitve_owner();
+ }
+ // This function check if it is possible to grow the reader count without
+ // overflowing the state.
+ LIBC_INLINE cpp::optional<State> try_increase_reader_count() const {
+ LIBC_ASSERT(!has_active_writer() &&
+ "try_increase_reader_count shall only be called when there "
+ "is no active writer.");
+ State res;
+ if (LIBC_UNLIKELY(__builtin_sadd_overflow(state, ACTIVE_READER_COUNT_UNIT,
+ &res.state)))
+ return cpp::nullopt;
+ return res;
+ }
+
+ // Utilities to do atomic operations on the state.
+ LIBC_INLINE static State
+ fetch_sub_reader_count(cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.fetch_sub(ACTIVE_READER_COUNT_UNIT, order));
+ }
+ LIBC_INLINE static State
+ load(cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.load(order));
+ }
+ LIBC_INLINE static State fetch_set_pending_reader(
+ cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.fetch_or(PENDING_READER_BIT, order));
+ }
+ LIBC_INLINE static State fetch_clear_pending_reader(
+ cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.fetch_and(~PENDING_READER_BIT, order));
+ }
+ LIBC_INLINE static State fetch_set_pending_writer(
+ cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.fetch_or(PENDING_WRITER_BIT, order));
+ }
+ LIBC_INLINE static State fetch_clear_pending_writer(
+ cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.fetch_and(~PENDING_WRITER_BIT, order));
+ }
+ LIBC_INLINE static State fetch_set_active_writer(
+ cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.fetch_or(ACTIVE_WRITER_BIT, order));
+ }
+ LIBC_INLINE static State fetch_clear_active_writer(
+ cpp::Atomic<Type> &target,
+ cpp::MemoryOrder order = cpp::MemoryOrder::SEQ_CST) {
+ return State(target.fetch_and(~ACTIVE_WRITER_BIT, order));
+ }
+
+ LIBC_INLINE bool
+ compare_exchange_weak_with(cpp::Atomic<Type> &target, State desired,
+ cpp::MemoryOrder success_order,
+ cpp::MemoryOrder failure_order) {
+ return target.compare_exchange_weak(state, desired, success_order,
+ failure_order);
+ }
+
+ // Utilities to spin and reload the state.
+ private:
+ template <class F>
+ LIBC_INLINE static State spin_reload_until(cpp::Atomic<Type> &target,
+ F &&func, unsigned spin_count) {
+ for (;;) {
+ auto state = State::load(target);
+ if (func(state) || spin_count == 0)
+ return state;
+ sleep_briefly();
+ spin_count--;
+ }
+ }
+
+ public:
+ // Return the reader state if either the lock is available or there is any
+ // ongoing contention.
+ LIBC_INLINE static State spin_reload_for_reader(cpp::Atomic<Type> &target,
+ Preference preference,
+ unsigned spin_count) {
+ return spin_reload_until(
+ target,
+ [=](State state) {
+ return state.can_acquire_reader(preference) || state.has_pending();
+ },
+ spin_count);
+ }
+ // Return the writer state if either the lock is available or there is any
+ // contention *between writers*. Since writers can be way less than readers,
+ // we allow them to spin more to improve the fairness.
+ LIBC_INLINE static State spin_reload_for_writer(cpp::Atomic<Type> &target,
+ Preference preference,
+ unsigned spin_count) {
+ return spin_reload_until(
+ target,
+ [=](State state) {
+ return state.can_acquire_writer(preference) ||
+ state.has_pending_writer();
+ },
+ spin_count);
+ }
+ };
+
+private:
+ // Whether the RwLock is shared between processes.
+ bool is_pshared;
+ // Reader/Writer preference.
+ Preference preference;
+ // State to keep track of the RwLock.
+ cpp::Atomic<int32_t> state;
+ // writer_tid is used to keep track of the thread id of the writer. Notice
+ // that TLS address is not a good idea here since it may remains the same
+ // across forked processes.
+ cpp::Atomic<pid_t> writer_tid;
+ // Waiting queue to keep track of the pending readers and writers.
+ WaitingQueue queue;
+
+private:
+ // TODO: use cached thread id once implemented.
+ LIBC_INLINE static pid_t gettid() { return syscall_impl<pid_t>(SYS_gettid); }
+
+ LIBC_INLINE LockResult try_read_lock(State &old) {
+ while (LIBC_LIKELY(old.can_acquire_reader(preference))) {
+ cpp::optional<State> next = old.try_increase_reader_count();
+ if (!next)
+ return LockResult::Overflow;
+ if (LIBC_LIKELY(old.compare_exchange_weak_with(
+ state, *next, cpp::MemoryOrder::ACQUIRE,
+ cpp::MemoryOrder::RELAXED)))
+ return LockResult::Success;
+ // Notice that old is updated by the compare_exchange_weak_with function.
+ }
+ return LockResult::Busy;
+ }
+
+ LIBC_INLINE LockResult try_write_lock(State &old) {
+ // This while loop should terminate quickly
+ while (LIBC_LIKELY(old.can_acquire_writer(preference))) {
+ if (LIBC_LIKELY(old.compare_exchange_weak_with(
+ state, old.set_writer_bit(), cpp::MemoryOrder::ACQUIRE,
+ cpp::MemoryOrder::RELAXED))) {
+ writer_tid.store(gettid(), cpp::MemoryOrder::RELAXED);
+ return LockResult::Success;
+ }
+ // Notice that old is updated by the compare_exchange_weak_with function.
+ }
+ return LockResult::Busy;
+ }
+
+public:
+ LIBC_INLINE constexpr RwLock(Preference preference = Preference::Reader,
+ bool is_pshared = false)
+ : is_pshared(is_pshared), preference(preference), state(0), writer_tid(0),
+ queue() {}
+
+ LIBC_INLINE LockResult try_read_lock() {
+ State old = State::load(state, cpp::MemoryOrder::RELAXED);
+ return try_read_lock(old);
+ }
+ LIBC_INLINE LockResult try_write_lock() {
+ State old = State::load(state, cpp::MemoryOrder::RELAXED);
+ return try_write_lock(old);
+ }
+
+private:
+ template <State (&SpinReload)(cpp::Atomic<int32_t> &, Preference, unsigned),
+ State (&SetPending)(cpp::Atomic<int32_t> &, cpp::MemoryOrder),
+ State (&ClearPending)(cpp::Atomic<int32_t> &, cpp::MemoryOrder),
+ FutexWordType &(WaitingQueue::Guard::*Serialization)(),
+ FutexWordType &(WaitingQueue::Guard::*PendingCount)(),
+ LockResult (RwLock::*TryLock)(State &),
+ long (WaitingQueue::*Wait)(FutexWordType,
+ cpp::optional<Futex::Timeout>, bool),
+ bool (State::*CanAcquire)(Preference) const>
----------------
QuarticCat wrote:
I think you can use a single `class T` and then implement these functions for a `Reader` type and a `Writer` type. Just like `std::priority_queue`'s `Compare` parameter. Or something like `std::allocator_traits` / `std::ranges::range` although I don't think we need to write so much for an internal function. Or simply write two versions. Honestly I prefer the last solution as I can't see too much value in abstracting it.
https://github.com/llvm/llvm-project/pull/94156
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