std/thread/current.rs
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use super::{Thread, ThreadId};
use crate::mem::ManuallyDrop;
use crate::ptr;
use crate::sys::thread_local::local_pointer;
const NONE: *mut () = ptr::null_mut();
const BUSY: *mut () = ptr::without_provenance_mut(1);
const DESTROYED: *mut () = ptr::without_provenance_mut(2);
local_pointer! {
static CURRENT;
}
/// Persistent storage for the thread ID.
///
/// We store the thread ID so that it never gets destroyed during the lifetime
/// of a thread, either using `#[thread_local]` or multiple `local_pointer!`s.
mod id {
use super::*;
cfg_if::cfg_if! {
if #[cfg(target_thread_local)] {
use crate::cell::Cell;
#[thread_local]
static ID: Cell<Option<ThreadId>> = Cell::new(None);
pub(super) const CHEAP: bool = true;
pub(super) fn get() -> Option<ThreadId> {
ID.get()
}
pub(super) fn set(id: ThreadId) {
ID.set(Some(id))
}
} else if #[cfg(target_pointer_width = "16")] {
local_pointer! {
static ID0;
static ID16;
static ID32;
static ID48;
}
pub(super) const CHEAP: bool = false;
pub(super) fn get() -> Option<ThreadId> {
let id0 = ID0.get().addr() as u64;
let id16 = ID16.get().addr() as u64;
let id32 = ID32.get().addr() as u64;
let id48 = ID48.get().addr() as u64;
ThreadId::from_u64((id48 << 48) + (id32 << 32) + (id16 << 16) + id0)
}
pub(super) fn set(id: ThreadId) {
let val = id.as_u64().get();
ID0.set(ptr::without_provenance_mut(val as usize));
ID16.set(ptr::without_provenance_mut((val >> 16) as usize));
ID32.set(ptr::without_provenance_mut((val >> 32) as usize));
ID48.set(ptr::without_provenance_mut((val >> 48) as usize));
}
} else if #[cfg(target_pointer_width = "32")] {
local_pointer! {
static ID0;
static ID32;
}
pub(super) const CHEAP: bool = false;
pub(super) fn get() -> Option<ThreadId> {
let id0 = ID0.get().addr() as u64;
let id32 = ID32.get().addr() as u64;
ThreadId::from_u64((id32 << 32) + id0)
}
pub(super) fn set(id: ThreadId) {
let val = id.as_u64().get();
ID0.set(ptr::without_provenance_mut(val as usize));
ID32.set(ptr::without_provenance_mut((val >> 32) as usize));
}
} else {
local_pointer! {
static ID;
}
pub(super) const CHEAP: bool = true;
pub(super) fn get() -> Option<ThreadId> {
let id = ID.get().addr() as u64;
ThreadId::from_u64(id)
}
pub(super) fn set(id: ThreadId) {
let val = id.as_u64().get();
ID.set(ptr::without_provenance_mut(val as usize));
}
}
}
#[inline]
pub(super) fn get_or_init() -> ThreadId {
get().unwrap_or_else(
#[cold]
|| {
let id = ThreadId::new();
id::set(id);
id
},
)
}
}
/// Tries to set the thread handle for the current thread. Fails if a handle was
/// already set or if the thread ID of `thread` would change an already-set ID.
pub(crate) fn set_current(thread: Thread) -> Result<(), Thread> {
if CURRENT.get() != NONE {
return Err(thread);
}
match id::get() {
Some(id) if id == thread.id() => {}
None => id::set(thread.id()),
_ => return Err(thread),
}
// Make sure that `crate::rt::thread_cleanup` will be run, which will
// call `drop_current`.
crate::sys::thread_local::guard::enable();
CURRENT.set(thread.into_raw().cast_mut());
Ok(())
}
/// Gets the id of the thread that invokes it.
///
/// This function will always succeed, will always return the same value for
/// one thread and is guaranteed not to call the global allocator.
#[inline]
pub(crate) fn current_id() -> ThreadId {
// If accessing the persistant thread ID takes multiple TLS accesses, try
// to retrieve it from the current thread handle, which will only take one
// TLS access.
if !id::CHEAP {
let current = CURRENT.get();
if current > DESTROYED {
unsafe {
let current = ManuallyDrop::new(Thread::from_raw(current));
return current.id();
}
}
}
id::get_or_init()
}
/// Gets a handle to the thread that invokes it, if the handle has been initialized.
pub(crate) fn try_current() -> Option<Thread> {
let current = CURRENT.get();
if current > DESTROYED {
unsafe {
let current = ManuallyDrop::new(Thread::from_raw(current));
Some((*current).clone())
}
} else {
None
}
}
/// Gets a handle to the thread that invokes it. If the handle stored in thread-
/// local storage was already destroyed, this creates a new unnamed temporary
/// handle to allow thread parking in nearly all situations.
pub(crate) fn current_or_unnamed() -> Thread {
let current = CURRENT.get();
if current > DESTROYED {
unsafe {
let current = ManuallyDrop::new(Thread::from_raw(current));
(*current).clone()
}
} else if current == DESTROYED {
Thread::new_unnamed(id::get_or_init())
} else {
init_current(current)
}
}
/// Gets a handle to the thread that invokes it.
///
/// # Examples
///
/// Getting a handle to the current thread with `thread::current()`:
///
/// ```
/// use std::thread;
///
/// let handler = thread::Builder::new()
/// .name("named thread".into())
/// .spawn(|| {
/// let handle = thread::current();
/// assert_eq!(handle.name(), Some("named thread"));
/// })
/// .unwrap();
///
/// handler.join().unwrap();
/// ```
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn current() -> Thread {
let current = CURRENT.get();
if current > DESTROYED {
unsafe {
let current = ManuallyDrop::new(Thread::from_raw(current));
(*current).clone()
}
} else {
init_current(current)
}
}
#[cold]
fn init_current(current: *mut ()) -> Thread {
if current == NONE {
CURRENT.set(BUSY);
// If the thread ID was initialized already, use it.
let id = id::get_or_init();
let thread = Thread::new_unnamed(id);
// Make sure that `crate::rt::thread_cleanup` will be run, which will
// call `drop_current`.
crate::sys::thread_local::guard::enable();
CURRENT.set(thread.clone().into_raw().cast_mut());
thread
} else if current == BUSY {
// BUSY exists solely for this check, but as it is in the slow path, the
// extra TLS write above shouldn't matter. The alternative is nearly always
// a stack overflow.
// If you came across this message, contact the author of your allocator.
// If you are said author: A surprising amount of functions inside the
// standard library (e.g. `Mutex`, `thread_local!`, `File` when using long
// paths, even `panic!` when using unwinding), need memory allocation, so
// you'll get circular dependencies all over the place when using them.
// I (joboet) highly recommend using only APIs from core in your allocator
// and implementing your own system abstractions. Still, if you feel that
// a particular API should be entirely allocation-free, feel free to open
// an issue on the Rust repository, we'll see what we can do.
rtabort!(
"\n\
Attempted to access thread-local data while allocating said data.\n\
Do not access functions that allocate in the global allocator!\n\
This is a bug in the global allocator.\n\
"
)
} else {
debug_assert_eq!(current, DESTROYED);
panic!(
"use of std::thread::current() is not possible after the thread's \
local data has been destroyed"
)
}
}
/// This should be run in [`crate::rt::thread_cleanup`] to reset the thread
/// handle.
pub(crate) fn drop_current() {
let current = CURRENT.get();
if current > DESTROYED {
unsafe {
CURRENT.set(DESTROYED);
drop(Thread::from_raw(current));
}
}
}