From 9622cdee1ad9657dd93d67a9ffdbc16e196ee4fa Mon Sep 17 00:00:00 2001 From: joboet Date: Sun, 11 Dec 2022 13:01:29 +0100 Subject: [PATCH] std: replace generic thread parker with explicit no-op parker --- library/std/src/sys/unsupported/mod.rs | 1 + .../std/src/sys/unsupported/thread_parking.rs | 11 ++ library/std/src/sys/wasi/mod.rs | 2 + library/std/src/sys/wasm/mod.rs | 2 + .../src/sys_common/thread_parking/generic.rs | 125 ------------------ .../std/src/sys_common/thread_parking/mod.rs | 5 +- 6 files changed, 17 insertions(+), 129 deletions(-) create mode 100644 library/std/src/sys/unsupported/thread_parking.rs delete mode 100644 library/std/src/sys_common/thread_parking/generic.rs diff --git a/library/std/src/sys/unsupported/mod.rs b/library/std/src/sys/unsupported/mod.rs index 15b22c620d583..e1a38de647116 100644 --- a/library/std/src/sys/unsupported/mod.rs +++ b/library/std/src/sys/unsupported/mod.rs @@ -22,6 +22,7 @@ pub mod thread; #[cfg(target_thread_local)] pub mod thread_local_dtor; pub mod thread_local_key; +pub mod thread_parking; pub mod time; mod common; diff --git a/library/std/src/sys/unsupported/thread_parking.rs b/library/std/src/sys/unsupported/thread_parking.rs new file mode 100644 index 0000000000000..197078bb18673 --- /dev/null +++ b/library/std/src/sys/unsupported/thread_parking.rs @@ -0,0 +1,11 @@ +use crate::pin::Pin; +use crate::time::Duration; + +pub struct Parker {} + +impl Parker { + pub unsafe fn new_in_place(_parker: *mut Parker) {} + pub unsafe fn park(self: Pin<&Self>) {} + pub unsafe fn park_timeout(self: Pin<&Self>, _dur: Duration) {} + pub fn unpark(self: Pin<&Self>) {} +} diff --git a/library/std/src/sys/wasi/mod.rs b/library/std/src/sys/wasi/mod.rs index 1dc3f2b20266d..c2c2f8a35deb8 100644 --- a/library/std/src/sys/wasi/mod.rs +++ b/library/std/src/sys/wasi/mod.rs @@ -49,6 +49,8 @@ pub mod thread; pub mod thread_local_dtor; #[path = "../unsupported/thread_local_key.rs"] pub mod thread_local_key; +#[path = "../unsupported/thread_parking.rs"] +pub mod thread_parking; pub mod time; #[path = "../unsupported/common.rs"] diff --git a/library/std/src/sys/wasm/mod.rs b/library/std/src/sys/wasm/mod.rs index 77ebe3c4ac6f5..6c05b56e1bfc0 100644 --- a/library/std/src/sys/wasm/mod.rs +++ b/library/std/src/sys/wasm/mod.rs @@ -70,6 +70,8 @@ cfg_if::cfg_if! { pub mod once; #[path = "../unsupported/thread.rs"] pub mod thread; + #[path = "../unsupported/thread_parking.rs"] + pub mod thread_parking; } } diff --git a/library/std/src/sys_common/thread_parking/generic.rs b/library/std/src/sys_common/thread_parking/generic.rs deleted file mode 100644 index 3209bffe353ed..0000000000000 --- a/library/std/src/sys_common/thread_parking/generic.rs +++ /dev/null @@ -1,125 +0,0 @@ -//! Parker implementation based on a Mutex and Condvar. - -use crate::pin::Pin; -use crate::sync::atomic::AtomicUsize; -use crate::sync::atomic::Ordering::SeqCst; -use crate::sync::{Condvar, Mutex}; -use crate::time::Duration; - -const EMPTY: usize = 0; -const PARKED: usize = 1; -const NOTIFIED: usize = 2; - -pub struct Parker { - state: AtomicUsize, - lock: Mutex<()>, - cvar: Condvar, -} - -impl Parker { - /// Construct the generic parker. The UNIX parker implementation - /// requires this to happen in-place. - pub unsafe fn new_in_place(parker: *mut Parker) { - parker.write(Parker { - state: AtomicUsize::new(EMPTY), - lock: Mutex::new(()), - cvar: Condvar::new(), - }); - } - - // This implementation doesn't require `unsafe` and `Pin`, but other implementations do. - pub unsafe fn park(self: Pin<&Self>) { - // If we were previously notified then we consume this notification and - // return quickly. - if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() { - return; - } - - // Otherwise we need to coordinate going to sleep - let mut m = self.lock.lock().unwrap(); - match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) { - Ok(_) => {} - Err(NOTIFIED) => { - // We must read here, even though we know it will be `NOTIFIED`. - // This is because `unpark` may have been called again since we read - // `NOTIFIED` in the `compare_exchange` above. We must perform an - // acquire operation that synchronizes with that `unpark` to observe - // any writes it made before the call to unpark. To do that we must - // read from the write it made to `state`. - let old = self.state.swap(EMPTY, SeqCst); - assert_eq!(old, NOTIFIED, "park state changed unexpectedly"); - return; - } // should consume this notification, so prohibit spurious wakeups in next park. - Err(_) => panic!("inconsistent park state"), - } - loop { - m = self.cvar.wait(m).unwrap(); - match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) { - Ok(_) => return, // got a notification - Err(_) => {} // spurious wakeup, go back to sleep - } - } - } - - // This implementation doesn't require `unsafe` and `Pin`, but other implementations do. - pub unsafe fn park_timeout(self: Pin<&Self>, dur: Duration) { - // Like `park` above we have a fast path for an already-notified thread, and - // afterwards we start coordinating for a sleep. - // return quickly. - if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() { - return; - } - let m = self.lock.lock().unwrap(); - match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) { - Ok(_) => {} - Err(NOTIFIED) => { - // We must read again here, see `park`. - let old = self.state.swap(EMPTY, SeqCst); - assert_eq!(old, NOTIFIED, "park state changed unexpectedly"); - return; - } // should consume this notification, so prohibit spurious wakeups in next park. - Err(_) => panic!("inconsistent park_timeout state"), - } - - // Wait with a timeout, and if we spuriously wake up or otherwise wake up - // from a notification we just want to unconditionally set the state back to - // empty, either consuming a notification or un-flagging ourselves as - // parked. - let (_m, _result) = self.cvar.wait_timeout(m, dur).unwrap(); - match self.state.swap(EMPTY, SeqCst) { - NOTIFIED => {} // got a notification, hurray! - PARKED => {} // no notification, alas - n => panic!("inconsistent park_timeout state: {n}"), - } - } - - // This implementation doesn't require `Pin`, but other implementations do. - pub fn unpark(self: Pin<&Self>) { - // To ensure the unparked thread will observe any writes we made - // before this call, we must perform a release operation that `park` - // can synchronize with. To do that we must write `NOTIFIED` even if - // `state` is already `NOTIFIED`. That is why this must be a swap - // rather than a compare-and-swap that returns if it reads `NOTIFIED` - // on failure. - match self.state.swap(NOTIFIED, SeqCst) { - EMPTY => return, // no one was waiting - NOTIFIED => return, // already unparked - PARKED => {} // gotta go wake someone up - _ => panic!("inconsistent state in unpark"), - } - - // There is a period between when the parked thread sets `state` to - // `PARKED` (or last checked `state` in the case of a spurious wake - // up) and when it actually waits on `cvar`. If we were to notify - // during this period it would be ignored and then when the parked - // thread went to sleep it would never wake up. Fortunately, it has - // `lock` locked at this stage so we can acquire `lock` to wait until - // it is ready to receive the notification. - // - // Releasing `lock` before the call to `notify_one` means that when the - // parked thread wakes it doesn't get woken only to have to wait for us - // to release `lock`. - drop(self.lock.lock().unwrap()); - self.cvar.notify_one() - } -} diff --git a/library/std/src/sys_common/thread_parking/mod.rs b/library/std/src/sys_common/thread_parking/mod.rs index e8e028bb3308f..c4d3f9ea2f427 100644 --- a/library/std/src/sys_common/thread_parking/mod.rs +++ b/library/std/src/sys_common/thread_parking/mod.rs @@ -18,10 +18,7 @@ cfg_if::cfg_if! { ))] { mod id; pub use id::Parker; - } else if #[cfg(any(windows, target_family = "unix"))] { - pub use crate::sys::thread_parking::Parker; } else { - mod generic; - pub use generic::Parker; + pub use crate::sys::thread_parking::Parker; } }