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//! A lock that supports one writer or many readers.
//! This API is for kernel threads, not evented I/O.
//! This API requires being initialized at runtime, and initialization
//! can fail. Once initialized, the core operations cannot fail.
impl: Impl = .{},
const RwLock = @This();
const std = @import("../std.zig");
const builtin = @import("builtin");
const assert = std.debug.assert;
pub const Impl = if (builtin.single_threaded)
SingleThreadedRwLock
else if (std.Thread.use_pthreads)
PthreadRwLock
else
DefaultRwLock;
/// Attempts to obtain exclusive lock ownership.
/// Returns `true` if the lock is obtained, `false` otherwise.
pub fn tryLock(rwl: *RwLock) bool {
return rwl.impl.tryLock();
}
/// Blocks until exclusive lock ownership is acquired.
pub fn lock(rwl: *RwLock) void {
return rwl.impl.lock();
}
/// Releases a held exclusive lock.
/// Asserts the lock is held exclusively.
pub fn unlock(rwl: *RwLock) void {
return rwl.impl.unlock();
}
/// Attempts to obtain shared lock ownership.
/// Returns `true` if the lock is obtained, `false` otherwise.
pub fn tryLockShared(rwl: *RwLock) bool {
return rwl.impl.tryLockShared();
}
/// Blocks until shared lock ownership is acquired.
pub fn lockShared(rwl: *RwLock) void {
return rwl.impl.lockShared();
}
/// Releases a held shared lock.
pub fn unlockShared(rwl: *RwLock) void {
return rwl.impl.unlockShared();
}
/// Single-threaded applications use this for deadlock checks in
/// debug mode, and no-ops in release modes.
pub const SingleThreadedRwLock = struct {
state: enum { unlocked, locked_exclusive, locked_shared } = .unlocked,
shared_count: usize = 0,
/// Attempts to obtain exclusive lock ownership.
/// Returns `true` if the lock is obtained, `false` otherwise.
pub fn tryLock(rwl: *SingleThreadedRwLock) bool {
switch (rwl.state) {
.unlocked => {
assert(rwl.shared_count == 0);
rwl.state = .locked_exclusive;
return true;
},
.locked_exclusive, .locked_shared => return false,
}
}
/// Blocks until exclusive lock ownership is acquired.
pub fn lock(rwl: *SingleThreadedRwLock) void {
assert(rwl.state == .unlocked); // deadlock detected
assert(rwl.shared_count == 0); // corrupted state detected
rwl.state = .locked_exclusive;
}
/// Releases a held exclusive lock.
/// Asserts the lock is held exclusively.
pub fn unlock(rwl: *SingleThreadedRwLock) void {
assert(rwl.state == .locked_exclusive);
assert(rwl.shared_count == 0); // corrupted state detected
rwl.state = .unlocked;
}
/// Attempts to obtain shared lock ownership.
/// Returns `true` if the lock is obtained, `false` otherwise.
pub fn tryLockShared(rwl: *SingleThreadedRwLock) bool {
switch (rwl.state) {
.unlocked => {
rwl.state = .locked_shared;
assert(rwl.shared_count == 0);
rwl.shared_count = 1;
return true;
},
.locked_exclusive, .locked_shared => return false,
}
}
/// Blocks until shared lock ownership is acquired.
pub fn lockShared(rwl: *SingleThreadedRwLock) void {
switch (rwl.state) {
.unlocked => {
rwl.state = .locked_shared;
assert(rwl.shared_count == 0);
rwl.shared_count = 1;
},
.locked_shared => {
rwl.shared_count += 1;
},
.locked_exclusive => unreachable, // deadlock detected
}
}
/// Releases a held shared lock.
pub fn unlockShared(rwl: *SingleThreadedRwLock) void {
switch (rwl.state) {
.unlocked => unreachable, // too many calls to `unlockShared`
.locked_exclusive => unreachable, // exclusively held lock
.locked_shared => {
rwl.shared_count -= 1;
if (rwl.shared_count == 0) {
rwl.state = .unlocked;
}
},
}
}
};
pub const PthreadRwLock = struct {
rwlock: std.c.pthread_rwlock_t = .{},
pub fn tryLock(rwl: *PthreadRwLock) bool {
return std.c.pthread_rwlock_trywrlock(&rwl.rwlock) == .SUCCESS;
}
pub fn lock(rwl: *PthreadRwLock) void {
const rc = std.c.pthread_rwlock_wrlock(&rwl.rwlock);
assert(rc == .SUCCESS);
}
pub fn unlock(rwl: *PthreadRwLock) void {
const rc = std.c.pthread_rwlock_unlock(&rwl.rwlock);
assert(rc == .SUCCESS);
}
pub fn tryLockShared(rwl: *PthreadRwLock) bool {
return std.c.pthread_rwlock_tryrdlock(&rwl.rwlock) == .SUCCESS;
}
pub fn lockShared(rwl: *PthreadRwLock) void {
const rc = std.c.pthread_rwlock_rdlock(&rwl.rwlock);
assert(rc == .SUCCESS);
}
pub fn unlockShared(rwl: *PthreadRwLock) void {
const rc = std.c.pthread_rwlock_unlock(&rwl.rwlock);
assert(rc == .SUCCESS);
}
};
pub const DefaultRwLock = struct {
state: usize = 0,
mutex: std.Thread.Mutex = .{},
semaphore: std.Thread.Semaphore = .{},
const IS_WRITING: usize = 1;
const WRITER: usize = 1 << 1;
const READER: usize = 1 << (1 + std.meta.bitCount(Count));
const WRITER_MASK: usize = std.math.maxInt(Count) << @ctz(usize, WRITER);
const READER_MASK: usize = std.math.maxInt(Count) << @ctz(usize, READER);
const Count = std.meta.Int(.unsigned, @divFloor(std.meta.bitCount(usize) - 1, 2));
pub fn tryLock(rwl: *DefaultRwLock) bool {
if (rwl.mutex.tryLock()) {
const state = @atomicLoad(usize, &rwl.state, .SeqCst);
if (state & READER_MASK == 0) {
_ = @atomicRmw(usize, &rwl.state, .Or, IS_WRITING, .SeqCst);
return true;
}
rwl.mutex.unlock();
}
return false;
}
pub fn lock(rwl: *DefaultRwLock) void {
_ = @atomicRmw(usize, &rwl.state, .Add, WRITER, .SeqCst);
rwl.mutex.lock();
const state = @atomicRmw(usize, &rwl.state, .Or, IS_WRITING, .SeqCst);
if (state & READER_MASK != 0)
rwl.semaphore.wait();
}
pub fn unlock(rwl: *DefaultRwLock) void {
_ = @atomicRmw(usize, &rwl.state, .And, ~IS_WRITING, .SeqCst);
rwl.mutex.unlock();
}
pub fn tryLockShared(rwl: *DefaultRwLock) bool {
const state = @atomicLoad(usize, &rwl.state, .SeqCst);
if (state & (IS_WRITING | WRITER_MASK) == 0) {
_ = @cmpxchgStrong(
usize,
&rwl.state,
state,
state + READER,
.SeqCst,
.SeqCst,
) orelse return true;
}
if (rwl.mutex.tryLock()) {
_ = @atomicRmw(usize, &rwl.state, .Add, READER, .SeqCst);
rwl.mutex.unlock();
return true;
}
return false;
}
pub fn lockShared(rwl: *DefaultRwLock) void {
var state = @atomicLoad(usize, &rwl.state, .SeqCst);
while (state & (IS_WRITING | WRITER_MASK) == 0) {
state = @cmpxchgWeak(
usize,
&rwl.state,
state,
state + READER,
.SeqCst,
.SeqCst,
) orelse return;
}
rwl.mutex.lock();
_ = @atomicRmw(usize, &rwl.state, .Add, READER, .SeqCst);
rwl.mutex.unlock();
}
pub fn unlockShared(rwl: *DefaultRwLock) void {
const state = @atomicRmw(usize, &rwl.state, .Sub, READER, .SeqCst);
if ((state & READER_MASK == READER) and (state & IS_WRITING != 0))
rwl.semaphore.post();
}
};
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