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runtime: only mitigate false sharing for multi-threaded runtimes
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,124 @@ | ||
| //! Helper module for allocating and deallocating tasks. | ||
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| use crate::runtime::task::core::{Cell, Core, Header, Trailer}; | ||
| use crate::runtime::task::Schedule; | ||
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| use std::alloc::{alloc, dealloc, handle_alloc_error, Layout}; | ||
| use std::future::Future; | ||
| use std::marker::PhantomData; | ||
| use std::mem::{align_of, size_of, ManuallyDrop}; | ||
| use std::ptr::{drop_in_place, NonNull}; | ||
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| fn layout_of<T: Future, S: Schedule>(scheduler: &S) -> Layout { | ||
| let size = std::mem::size_of::<Cell<T, S>>(); | ||
| let mut align = std::mem::align_of::<Cell<T, S>>(); | ||
| let min_align = scheduler.min_align(); | ||
| if align < min_align { | ||
| align = min_align; | ||
| } | ||
| match Layout::from_size_align(size, align) { | ||
| Ok(layout) => layout, | ||
| Err(_) => panic!("Failed to build layout of type."), | ||
| } | ||
| } | ||
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|
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| /// A `Box<Cell<T, S>>` with an alignment of at least `s.min_align()`. | ||
| pub(super) struct TaskBox<T: Future, S: Schedule> { | ||
| ptr: NonNull<Cell<T, S>>, | ||
| _phantom: PhantomData<Cell<T, S>>, | ||
| } | ||
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| impl<T: Future, S: Schedule> TaskBox<T, S> { | ||
| /// Creates a new task allocation. | ||
| pub(super) fn new(header: Header, core: Core<T, S>) -> Self { | ||
| let layout = layout_of::<T, S>(&core.scheduler); | ||
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| assert_eq!(size_of::<Cell<T, S>>(), layout.size()); | ||
| assert_ne!(size_of::<Cell<T, S>>(), 0); | ||
| assert!(align_of::<Cell<T, S>>() <= layout.align()); | ||
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|
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| // SAFETY: The size of `layout` is non-zero as checked above. | ||
| let ptr = unsafe { alloc(layout) } as *mut Cell<T, S>; | ||
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| let ptr = match NonNull::new(ptr) { | ||
| Some(ptr) => ptr, | ||
| None => handle_alloc_error(layout), | ||
| }; | ||
|
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| // SAFETY: We just allocated memory with the same size and a compatible | ||
| // alignment for `Cell<T, S>`. | ||
| unsafe { | ||
| ptr.as_ptr().write(Cell { | ||
| header, | ||
| core, | ||
| trailer: Trailer::new(), | ||
| }); | ||
| }; | ||
|
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||
| Self { | ||
| ptr, | ||
| _phantom: PhantomData, | ||
| } | ||
| } | ||
|
|
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| /// Convert this allocation into a raw pointer. | ||
| pub(super) fn into_raw(self) -> NonNull<Cell<T, S>> { | ||
| let me = ManuallyDrop::new(self); | ||
| me.ptr | ||
| } | ||
|
|
||
| /// Convert this allocation back into a `TaskBox`. | ||
| /// | ||
| /// # Safety | ||
| /// | ||
| /// The provided pointer must originate from a previous call to `into_raw`, | ||
| /// and the raw pointer must not be used again after this call. | ||
| pub(super) unsafe fn from_raw(ptr: NonNull<Cell<T, S>>) -> Self { | ||
| Self { | ||
| ptr, | ||
| _phantom: PhantomData, | ||
| } | ||
| } | ||
| } | ||
|
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| impl<T: Future, S: Schedule> std::ops::Deref for TaskBox<T, S> { | ||
| type Target = Cell<T, S>; | ||
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| fn deref(&self) -> &Cell<T, S> { | ||
| // SAFETY: This box always points at a valid cell. | ||
| unsafe { &*self.ptr.as_ptr() } | ||
| } | ||
| } | ||
|
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| impl<T: Future, S: Schedule> Drop for TaskBox<T, S> { | ||
| fn drop(&mut self) { | ||
| let ptr = self.ptr.as_ptr(); | ||
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| // SAFETY: The task is still valid, so we can dereference the pointer. | ||
| let layout = layout_of::<T, S>(unsafe { &(*ptr).core.scheduler }); | ||
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| // SAFETY: The pointer was allocated with this layout. (The return value | ||
| // of `min_align` doesn't change.) | ||
| let _drop_helper = DropHelper { | ||
| layout, | ||
| ptr: ptr as *mut u8, | ||
| }; | ||
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| // SAFETY: A task box contains a pointer to a valid cell, and we have | ||
| // not dropped the allocation yet. | ||
| unsafe { drop_in_place(self.ptr.as_ptr()) }; | ||
| } | ||
| } | ||
|
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| struct DropHelper { | ||
| ptr: *mut u8, | ||
| layout: Layout, | ||
| } | ||
|
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| impl Drop for DropHelper { | ||
| #[inline] | ||
| fn drop(&mut self) { | ||
| // SAFETY: See `TaskBox::drop`. | ||
| unsafe { dealloc(self.ptr, self.layout) }; | ||
| } | ||
| } |
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