Explaining 200 Line Lightweight Streams in Rust

Lightweight threads (coroutines, coroutines, green threads) are a very powerful mechanism in modern programming languages. In this article, Carl Fredrik Samson tried to implement a runtime for lightweight streams on Rast, along the way explaining how they work under the hood.

It should also be noted that the article is not super fresh, so in order for the examples to work in the modern nightly version of the Rust compiler, you will most likely need changes that can be found in the code repository for this article.

He translated for himself for the most part. Write about all the comments - I will correct it promptly. I tried to translate the text closely, but in some places I reformulated it to make it easier to read and understand.

Green Threads

Green threads solve a common problem in programming - you don't want your code to block the processor, wasting resources. This is solved by using multitasking, which allows you to pause the execution of one piece of code, launching another for execution, switching between "contexts".

, — . — . , .

()

, . — "-" ( - ). , , UI (User Interface — ) , . , , , .

. , - , , - . (yielding

control) . - , - . , /. , , - , .

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-, . , .. . , .

— x86-64. 16 :

, "callee saved". , — , , .. .

, . , . :

mov %rsp, %rax

. . , , ( )

: AT&T .

AT&T Rust. , . LLVM. LLVM , .

AT&T.

. , . %rax

, :

%rax    # 64   (8 )
%eax    #  32   "rax"
%ax     #  16   "rax"
%ah     #  8   "ax"  "rax"
%al     #  8   "rax"

+-----------------------------------------------------------------------+
|76543210 76543210 76543210 76543210 76543210 76543210 76543210 76543210|
+--------+--------+--------+--------+--------+--------+--------+--------+
|        |        |        |        |        |        |   %ah  |   %al  |
+--------+--------+--------+--------+--------+--------+--------+--------+
|        |        |        |        |        |        |       %ax       |
+--------+--------+--------+--------+--------+--------+-----------------+
|        |        |        |        |               %eax                |
+--------+--------+--------+--------+-----------------------------------+
|                                 %rax                                  |
+-----------------------------------------------------------------------+

, . 64, 64 .

"" . , 16 , 16 . , .. AT&T q

(quad-word — ) l

(long-word — ). movq

4 * 16 = 64 .

mov

, . AT&T.

16 x86-64. .

,

, .

"green_threads".

cargo init

- , , :

rustup override set nightly

main.rs

, llvm_asm!

:

#![feature(llvm_asm)]

, 48 , .

const SSIZE: isize = 48;

, . , , , , .

#[derive(Debug, Default)]
#[repr(C)]
struct ThreadContext {
    rsp: u64,
}

, "callee saved" , . ABI x86-64. , .

, - , #[repr(C)]

. ABI, , rsp

8 . ABI, .

fn hello() -> ! {
    println!("I LOVE WAKING UP ON A NEW STACK!");

    loop {}
}

, , .

, :

unsafe fn gt_switch(new: *const ThreadContext) {
    llvm_asm!("
        mov     0x00($0), %rsp
        ret
    "
    :
    : "r"(new)
    :
    : "alignstack" //    ,    
    );
}

. , . , rsp

( new.rsp

, , ). ?

ret

, . %rsp

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, .

, , .

, . . , :

unsafe

— , , . , .

gt_switch(new: *const ThreadContext)

— ThreadContext

, .

llvm_asm!("

— . , - AT&T (-).

, — — mov 0x00($0), %rsp

. , , 0x00 ( ; ) $0 , rsp

. rsp

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input

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.

$

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:

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: "r"(new)

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clobber list
:

— clobber list

— , , , , . , , , , . .. , .

options
: "alignstack"

— . : alignstack

, volatile

intel

. , . Windows alignstack

.

fn main() {
    let mut ctx = ThreadContext::default();
    let mut stack = vec![0_u8; SSIZE as usize];

    unsafe {
        let stack_bottom = stack.as_mut_ptr().offset(SSIZE);
        let sb_aligned = (stack_bottom as usize & !15) as *mut u8;
        std::ptr::write(sb_aligned.offset(-16) as *mut u64, hello as u64);
        ctx.rsp = sb_aligned.offset(-16) as u64;
        gt_switch(&mut ctx);
    }
}

. hello

( ), u64

, 64 , .

, — ( ). 48 0 47, 32 16 / .

|0          1           2          3           4       |4  5
|0123456789 012345|6789 0123456789 01|23456789 01234567|89 0123456789
|                 |                  |XXXXXXXX         |
|                 |                  |                 stack bottom
|0th byte         |16th byte         |32nd byte

, 16 (, 16 ?)

let sb_aligned = (stack_bottom as usize & !15) as *mut u8;

? Vec<u8>

, , 16 . , 16 . , .

, u64

u8

. u64

32-39, 8 . u64

- 32, , .

rsp

(Stack Pointer — ) 32 . u64

-, , .

cargo run

, :

dau@dau-work-pc:~/Projects/rust-programming-book/green_threads/green_threads$ cargo run
   Compiling green_threads v0.1.0 (/home/dau/Projects/rust-programming-book/green_threads/green_threads)
    Finished dev [unoptimized + debuginfo] target(s) in 0.44s
     Running `target/debug/green_threads`
I LOVE WAKING UP ON A NEW STACK!

? hello

, . : . .

. . "" " " — .

, . (push/pop) , . .

— , , Rust Programming Language.

. 64 8 . , u8

, , , 000

, 0008

0016

.

, .

stack pointer

, 16 , , , , 16 ( — ). , , , 0008

(, ).

( gt_switch

), .

: — hello

, , .

print!(
    "hello func address: 0x{addr:016X} ({addr})\n\n",
    addr = hello as usize
);

for i in (0..SSIZE).rev() {
    print!(
        "mem: {}, value: 0x{:02X}\n{}",
        stack.as_ptr().offset(i as isize) as usize,
        *stack.as_ptr().offset(i as isize),
        if i % 8 == 0 { "\n" } else { "" }
    );
}

hello func address: 0x0000560CD80B50B0 (94613164216496)

mem: 94613168839439, value: 0x00
mem: 94613168839438, value: 0x00
mem: 94613168839437, value: 0x00
mem: 94613168839436, value: 0x00
mem: 94613168839435, value: 0x00
mem: 94613168839434, value: 0x00
mem: 94613168839433, value: 0x00
mem: 94613168839432, value: 0x00

mem: 94613168839431, value: 0x00
mem: 94613168839430, value: 0x00
mem: 94613168839429, value: 0x56
mem: 94613168839428, value: 0x0C
mem: 94613168839427, value: 0xD8
mem: 94613168839426, value: 0x0B
mem: 94613168839425, value: 0x50
mem: 94613168839424, value: 0xB0

mem: 94613168839423, value: 0x00
mem: 94613168839422, value: 0x00
mem: 94613168839421, value: 0x00
mem: 94613168839420, value: 0x00
mem: 94613168839419, value: 0x00
mem: 94613168839418, value: 0x00
mem: 94613168839417, value: 0x00
mem: 94613168839416, value: 0x00

mem: 94613168839415, value: 0x00
mem: 94613168839414, value: 0x00
mem: 94613168839413, value: 0x00
mem: 94613168839412, value: 0x00
mem: 94613168839411, value: 0x00
mem: 94613168839410, value: 0x00
mem: 94613168839409, value: 0x00
mem: 94613168839408, value: 0x00

mem: 94613168839407, value: 0x00
mem: 94613168839406, value: 0x00
mem: 94613168839405, value: 0x00
mem: 94613168839404, value: 0x00
mem: 94613168839403, value: 0x00
mem: 94613168839402, value: 0x00
mem: 94613168839401, value: 0x00
mem: 94613168839400, value: 0x00

mem: 94613168839399, value: 0x00
mem: 94613168839398, value: 0x00
mem: 94613168839397, value: 0x00
mem: 94613168839396, value: 0x00
mem: 94613168839395, value: 0x00
mem: 94613168839394, value: 0x00
mem: 94613168839393, value: 0x00
mem: 94613168839392, value: 0x00

I LOVE WAKING UP ON A NEW STACK!

u64

, , ( — .).

, , , , 94613168839392

94613168839439

.

94613168839424

94613168839431

. — stack pointer

, , %rsp%

. , . ( — !!!)

0xB0, 0x50, 0x0B, 0xD8, 0x0C, 0x56, 0x00, 0x00

— ( ) hello()

, u8

.

, , 48 , , , .

, 8 , . , , , . " " (stack overflow), .

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, , - , . - , push()

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, , . , .

Windows x86-64 , x86-64 psABI. Windows , , , , , .

psABI :

, %rsp

— . , , base pointer, 16. 8 , , , . , - .

, stack_ptr + SSIZE - 16

, . - SSIZE

— .

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16 , ABI.

- , stack_ptr + SSIZE - 16

. :

, stack_ptr + SSIZE

( 16 ), .. , .
, stack_ptr + SSIZE - 8

, , 16 .

stack_ptr + SSIZE - 16

. 8 -16, -15, -14, ..., -9

(, , bottom of stack, .. ( — , , — , )).

, , , .

, , 1024 , , . .

, , . : BEFORE.txt

( ) AFTER.txt

( ). , .

- , — .

#![feature(llvm_asm)]
#![feature(naked_functions)]

use std::io::Write;

const SSIZE: isize = 1024;
static mut S_PTR: *const u8 = 0 as *const u8;

#[derive(Debug, Default)]
#[repr(C)]
struct ThreadContext {
    rsp: u64,
    r15: u64,
    r14: u64,
    r13: u64,
    r12: u64,
    rbx: u64,
    rbp: u64,
}

fn print_stack(filename: &str) {
    let mut f = std::fs::File::create(filename).unwrap();
    unsafe {
        for i in (0..SSIZE).rev() {
            writeln!(
                f,
                "mem: {}, val: {}",
                S_PTR.offset(i as isize) as usize,
                *S_PTR.offset( i as isize)
            )
            .expect("Error writing to file.");
        }
    }
}

fn hello() {
    println!("I LOVE WAKING UP ON A NEW STACK!");
    print_stack("AFTER.txt");

    loop {}
}

unsafe fn gt_switch(new: *const ThreadContext) {
    llvm_asm!("
        mov     0x00($0), %rsp
        ret
        "
        :
        : "r"(new)
        :
        : "alignstack"
    );
}

fn main() {

    let mut ctx = ThreadContext::default();
    let mut stack = vec![0_u8; SSIZE as usize];
    let stack_ptr = stack.as_mut_ptr();

    unsafe {
        S_PTR = stack_ptr;
        std::ptr::write(stack_ptr.offset(SSIZE - 16) as *mut u64, hello as u64);
        print_stack("BEFORE.txt");
        ctx.rsp = stack_ptr.offset(SSIZE - 16) as u64;
        gt_switch(&mut ctx);
    }
}

, , , , , , " " (best practicies) . , , , - , , , PR .

, — main.rs

, , :

#![feature(llvm_asm)]
#![feature(naked_functions)]

use std::ptr;

const DEFAULT_STACK_SIZE: usize = 1024 * 1024 * 2;
const MAX_THREADS: usize = 4;

static mut RUNTIME: usize = 0;

: asm

naked_functions

, .

`naked_functions`

, "" "", - , . , , . #[naked]

. .

, naked_functions

RFC #1201.

Naked- . , , , .. naked- . ret

naked- ( , ABI), . .

DEFAULT_STACK_SIZE

2 , , . (MAX_THREADS

) 4, .. .

— RUNTIME

— , (, , , , ).

- :

pub struct Runtime {
    threads: Vec<Thread>,
    current: usize,
}

#[derive(Debug, Eq, PartialEq)]
enum State {
    Available,
    Running,
    Ready,
}

struct Thread {
    id: usize,
    stack: Vec<u8>,
    ctx: ThreadContext,
    state: State,
}

#[derive(Debug, Default)]
#[repr(C)]
struct ThreadContext {
    rps: u64,
    r15: u64,
    r14, u64,
    r13: u64,
    r12: u64,
    rbx: u64,
    rbp: u64,
}

Runtime

. , . Thread

current

, , .

Thread

. , , id

. stack

, . ctx

, , . state

— .

State

— , :

Available

— , .
Running

—
Ready

— .

ThreadContext

, .

, " " , . x86-64 "callee saved".

impl Thread {
    fn new(id: usize) -> Self {
        Thread {
            id,
            stack: vec![0_u8; DEFAULT_STACK_SIZE],
            ctx: ThreadContext::default(),
            state: State::Available,
        }
    }
}

. Available

, , .

, . , .. . , , .

, , , . push()

, . , .

, Vec<T>

into_boxed_slice()

, Box<[T]>

— , . , .

impl Runtime

, .

impl Runtime {

    pub fn new() -> Self {
        let base_thread = Thread {
            id: 0,
            stack: vec![0_u8; DEFAULT_STACK_SIZE],
            ctx: ThreadContext::default(),
            state: State::Running,
        };

        let mut threads = vec![base_thread];
        let mut available_threads: Vec<Thread> = (1..MAX_THREADS).map(|i| Thread::new(i)).collect();
        threads.append(&mut available_threads);

        Runtime {
            threads,
            current: 0,
        }
    }

    // code of other methods is here
    // ...
}

Runtime

Running

. . , 0

, .

// ,       Runtime
//   ,      yield
//        .
//        
pub fn init(&self) {
    unsafe {
        let r_ptr: *const Runtime = self;
        RUNTIME = r_ptr as usize;
    }
}

. , , , yield

. , , , .

pub fn run(&mut self) -> ! {
    while self.t_yield() {};
    std::process::exit(0);
}

, . t_yield()

, false

, , , .

fn t_return(&mut self) {
    if self.current != 0 {
        std.threads[self.current].state = State::Available;
        self.t_yield();
    }
}

, . t_return

, .. return

. , — , , .

, . yield

. (spawned) , , , .. guard

( ). , t_return

— guard

.

Available

, , (task), t_yield

, .

yield

:

fn t_yield(&mut self) -> bool {
    let mut post = self.current;
    while self.threads[pos].state != State::Ready {
        pos += 1;
        if pos == self.threads.len() {
            pos = 0;
        }
        if pos == self.current {
            return false;
        }
    }

    if self.threads[self.current].state != State::Available {
        self.threads[self.current].state = State::Ready;
    }

    self.threads[pos].state = State::Running;
    let old_pos = self.current;
    self.current = pos;

    unsafe {
        let old: *mut ThreadContext = &mut self.threads[old_pos].ctx;
        let new: *const ThreadContext = &self.threads[pos].ctx;

        llvm_asm!(
            "
            mov $0, %rdi
            mov $1, %rsi
            "
            :
            : "r"(old), "r"(new)
            :
            :
        );
        switch();
    }

    self.threads.len() > 0
}

. t_yield

, .. yield

(. — , ).

, - Ready

, , , . , .

Ready

, . , (round-robin). , .

. , ( Ready

) -, , ?

. , , (poll) . , IsReady

, , Pending

, - . Ready

, . ? , , , .

, , Running

Ready

.

switch

, . , , .

naked

naked . , . , , , . , #[naked]

, . "" "" ThreadContext

, . Linux %rdi

, — %rsi

.

self.threads.len() > 0

— , . Windows, Linux, , . std::hint::black_box

, , , , . , , . , .

spawn()

:

pub fn spawn(&mut self, f: fn()) {
    let available = self
        .threads
        .iter_mut()
        .find(|t| t.state == State::Available)
        .expect("no available thread.");

    let size == available.stack.len();

    unsafe {
        let s_ptr = available.stack.as_mut_ptr().offset(size as isize);
        let s_ptr = (s_ptr as usize & !15) as *mut u8;
        std::ptr::write(s_ptr.offset(-16) as *mut u64, guard as u64);
        std::ptr::write(s_ptr.offset(-24) as *mut u64, skip as u64);
        std::ptr::write(s_ptr.offset(-32) as *mut u64, f as u64);
        available.ctx.rsp = s_ptr.offset(-32) as u64;
    }
    available.state = State::Ready;
}

//       `impl Runtime`

, t_yield

, spawn

.

, , , , , psABI.

, , (.. Available

). , , , , . .

, ( u8

) .

unsafe-. , 16 . guard

, , . skip

, , f

, guard

16 . , , f

.

, . , f

, . base pointer . skip

guard

. guard

16 , ABI.

, , rsp

( ) . .

, Ready

, , , . , "" .

. , , . , , .

`guard`

, `skip`

`switch`

fn guard() {
    unsafe {
        let rt_ptr = RUNTIME as *mut Runtime;
        (*rt_ptr).t_return();
    };
}

, , , , , , t_return()

. , , , , t_return

, . Available

( ), t_yield

, .

#[naked]
fn skip() { }

skip

. #[naked]

, ret

, , . , guard

.

pub fn yield_thread() {
    unsafe {
        let rt_ptr = RUNTIME as *mut Runtime;
        (*rt_ptr).t_yield();
    };
}

, t_yield

. , , (), . , , .

, . , .

#[naked]
#[inline(never)]
unsafe fn switch() {
    llvm_asm!("
        mov     %rsp, 0x00(%rdi)
        mov     %r15, 0x08(%rdi)
        mov     %r14, 0x10(%rdi)
        mov     %r13, 0x18(%rdi)
        mov     %r12, 0x20(%rdi)
        mov     %rbx, 0x28(%rdi)
        mov     %rbp, 0x30(%rdi)

        mov     0x00(%rsi), %rsp
        mov     0x08(%rsi), %r15
        mov     0x10(%rsi), %r14
        mov     0x18(%rsi), %r13
        mov     0x20(%rsi), %r12
        mov     0x28(%rsi), %rbx
        mov     0x30(%rsi), %rbp
        "
    );
}

. , , , , , "" .

, , .

#[naked]

. , , , . , .

. #[inline(never)]

, . , , --release

.

`main`

fn main() {
    let mut runtime = Runtime::new();

    runtime.init();

    runtime.spawn(|| {
        println!("THREAD 1 STARTING");
        let id = 1;
        for i in 1..=10 {
            println!("thread: {} counter: {}", id, i);
            yield_thread();
        }
        println!("THREAD 1 FINISHED");
    });

    runtime.spawn(|| {
        println!("THREAD 2 STARTING");
        let id = 2;
        for i in 1..=15 {
            println!("thread: {} counter: {}", id i);
            yield_thread();
        }
        println!("THREAD 2 FINISHED");
    });

    runtime.run();
}

, , 0 9 15, . cargo run

, :

THREAD 1 STARTING
thread: 1 counter: 1
THREAD 2 STARTING
thread: 2 counter: 1
thread: 1 counter: 2
thread: 2 counter: 2
thread: 1 counter: 3
thread: 2 counter: 3
thread: 1 counter: 4
thread: 2 counter: 4
thread: 1 counter: 5
thread: 2 counter: 5
thread: 1 counter: 6
thread: 2 counter: 6
thread: 1 counter: 7
thread: 2 counter: 7
thread: 1 counter: 8
thread: 2 counter: 8
thread: 1 counter: 9
thread: 2 counter: 9
thread: 1 counter: 10
thread: 2 counter: 10
THREAD 1 FINISHED
thread: 2 counter: 11
thread: 2 counter: 12
thread: 2 counter: 13
thread: 2 counter: 14
thread: 2 counter: 15
THREAD 2 FINISHED

. , . 1 , 2 .

, . , , . !

Explaining 200 Line Lightweight Streams in Rust

Explaining 200 Line Lightweight Streams in Rust

Green Threads

,

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naked

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