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const std = @import("index.zig");
const builtin = @import("builtin");
const AtomicOrder = builtin.AtomicOrder;
const AtomicRmwOp = builtin.AtomicRmwOp;
const assert = std.debug.assert;
const SpinLock = std.SpinLock;
const linux = std.os.linux;
const windows = std.os.windows;
/// Lock may be held only once. If the same thread
/// tries to acquire the same mutex twice, it deadlocks.
/// The Linux implementation is based on mutex3 from
/// https://www.akkadia.org/drepper/futex.pdf
pub const Mutex = switch(builtin.os) {
builtin.Os.linux => MutexLinux,
builtin.Os.windows => MutexWindows,
else => MutexSpinLock,
};
const MutexLinux = struct {
/// 0: unlocked
/// 1: locked, no waiters
/// 2: locked, one or more waiters
lock: i32,
pub const Held = struct {
mutex: *Mutex,
pub fn release(self: Held) void {
const c = @atomicRmw(i32, &self.mutex.lock, AtomicRmwOp.Sub, 1, AtomicOrder.Release);
if (c != 1) {
_ = @atomicRmw(i32, &self.mutex.lock, AtomicRmwOp.Xchg, 0, AtomicOrder.Release);
const rc = linux.futex_wake(&self.mutex.lock, linux.FUTEX_WAKE | linux.FUTEX_PRIVATE_FLAG, 1);
switch (linux.getErrno(rc)) {
0 => {},
linux.EINVAL => unreachable,
else => unreachable,
}
}
}
};
pub fn init() Mutex {
return Mutex {
.lock = 0,
};
}
pub fn deinit(self: *Mutex) void {}
pub fn acquire(self: *Mutex) Held {
var c = @cmpxchgWeak(i32, &self.lock, 0, 1, AtomicOrder.Acquire, AtomicOrder.Monotonic) orelse
return Held{ .mutex = self };
if (c != 2)
c = @atomicRmw(i32, &self.lock, AtomicRmwOp.Xchg, 2, AtomicOrder.Acquire);
while (c != 0) {
const rc = linux.futex_wait(&self.lock, linux.FUTEX_WAIT | linux.FUTEX_PRIVATE_FLAG, 2, null);
switch (linux.getErrno(rc)) {
0, linux.EINTR, linux.EAGAIN => {},
linux.EINVAL => unreachable,
else => unreachable,
}
c = @atomicRmw(i32, &self.lock, AtomicRmwOp.Xchg, 2, AtomicOrder.Acquire);
}
return Held { .mutex = self };
}
};
const MutexWindows = struct {
lock: ?windows.RTL_CRITICAL_SECTION,
pub const Held = struct {
mutex: *Mutex,
pub fn release(self: Held) void {
windows.LeaveCriticalSection(&self.mutex.lock);
}
};
fn initOsData(self: *MutexWindows) void {
if (self.lock == null) {
windows.InitializeCriticalSection(&self.lock);
}
}
pub fn init() Mutex {
return Mutex {
.lock = null,
};
}
pub fn deinit(self: *Mutex) void {
windows.DeleteCriticalSection(&self.lock);
self.lock = null;
}
pub fn acquire(self: *Mutex) Held {
self.initOsData();
windows.EnterCriticalSection(&self.lock);
return Held { .mutex = self };
}
};
const MutexSpinLock = struct {
/// TODO better implementation than spin lock
lock: SpinLock,
pub const Held = struct {
mutex: *Mutex,
pub fn release(self: Held) void {
SpinLock.Held.release(SpinLock.Held { .spinlock = &self.mutex.lock });
}
};
pub fn init() Mutex {
return Mutex {
.lock = SpinLock.init(),
};
}
pub fn deinit(self: *Mutex) void {}
pub fn acquire(self: *Mutex) Held {
_ = self.lock.acquire();
return Held { .mutex = self };
}
};
const Context = struct {
mutex: *Mutex,
data: i128,
const incr_count = 10000;
};
test "std.Mutex" {
var direct_allocator = std.heap.DirectAllocator.init();
defer direct_allocator.deinit();
var plenty_of_memory = try direct_allocator.allocator.alloc(u8, 300 * 1024);
defer direct_allocator.allocator.free(plenty_of_memory);
var fixed_buffer_allocator = std.heap.ThreadSafeFixedBufferAllocator.init(plenty_of_memory);
var a = &fixed_buffer_allocator.allocator;
var mutex = Mutex.init();
defer mutex.deinit();
var context = Context{
.mutex = &mutex,
.data = 0,
};
const thread_count = 10;
var threads: [thread_count]*std.os.Thread = undefined;
for (threads) |*t| {
t.* = try std.os.spawnThread(&context, worker);
}
for (threads) |t|
t.wait();
std.debug.assertOrPanic(context.data == thread_count * Context.incr_count);
}
fn worker(ctx: *Context) void {
var i: usize = 0;
while (i != Context.incr_count) : (i += 1) {
const held = ctx.mutex.acquire();
defer held.release();
ctx.data += 1;
}
}
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