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Fix Bytes when Vec pointer's LSB is set (#346)

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This separates the `SharedVtable` into 3:

- `PromotableEvenVtable`: The original `SharedVtable`, which will
  promote the `Vec` to `Shared` on the first clone, and is selected when
  the `Vec`'s pointer has the LSB unset.
- `PromotableOddVtable`: Similar to the `PromotableEvenVtable`, but
  selected when the `Vec`'s pointer has the LSB set. This vtable differs
  in the masking used when reconstructing the `Vec`.
- `SharedVtable`: This no longer checks if its current kind is `VEC` or
  `ARC`, and is only created by the "promotable" vtables.

This also adds a test using an "odd" global allocator that purposefully
bumps all pointers with alignment of 1.

Closes #343
This commit is contained in:
Sean McArthur
2019-12-17 13:23:15 -08:00
committed by GitHub
parent 8ae3bb2104
commit c7cf716180
2 changed files with 155 additions and 51 deletions
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src/bytes.rs
+88 -51
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@@ -731,20 +731,23 @@ impl From<Vec<u8>> for Bytes {
let slice = vec.into_boxed_slice();
let len = slice.len();
let ptr = slice.as_ptr();
assert!(
ptr as usize & KIND_VEC == 0,
"Vec pointer should not have LSB set: {:p}",
ptr,
);
drop(Box::into_raw(slice));
let data = ptr as usize | KIND_VEC;
Bytes {
ptr,
len,
data: AtomicPtr::new(data as *mut _),
vtable: &SHARED_VTABLE,
if ptr as usize & 0x1 == 0 {
let data = ptr as usize | KIND_VEC;
Bytes {
ptr,
len,
data: AtomicPtr::new(data as *mut _),
vtable: &PROMOTABLE_EVEN_VTABLE,
}
} else {
Bytes {
ptr,
len,
data: AtomicPtr::new(ptr as *mut _),
vtable: &PROMOTABLE_ODD_VTABLE,
}
}
}
}
@@ -782,6 +785,74 @@ unsafe fn static_drop(_: &mut AtomicPtr<()>, _: *const u8, _: usize) {
// nothing to drop for &'static [u8]
}
// ===== impl PromotableVtable =====
static PROMOTABLE_EVEN_VTABLE: Vtable = Vtable {
clone: promotable_even_clone,
drop: promotable_even_drop,
};
static PROMOTABLE_ODD_VTABLE: Vtable = Vtable {
clone: promotable_odd_clone,
drop: promotable_odd_drop,
};
unsafe fn promotable_even_clone(data: &AtomicPtr<()>, ptr: *const u8, len: usize) -> Bytes {
let shared = data.load(Ordering::Acquire);
let kind = shared as usize & KIND_MASK;
if kind == KIND_ARC {
shallow_clone_arc(shared as _, ptr, len)
} else {
debug_assert_eq!(kind, KIND_VEC);
let buf = (shared as usize & !KIND_MASK) as *mut u8;
shallow_clone_vec(data, shared, buf, ptr, len)
}
}
unsafe fn promotable_even_drop(data: &mut AtomicPtr<()>, ptr: *const u8, len: usize) {
let shared = *data.get_mut();
let kind = shared as usize & KIND_MASK;
if kind == KIND_ARC {
release_shared(shared as *mut Shared);
} else {
debug_assert_eq!(kind, KIND_VEC);
let buf = (shared as usize & !KIND_MASK) as *mut u8;
drop(rebuild_vec(buf, ptr, len));
}
}
unsafe fn promotable_odd_clone(data: &AtomicPtr<()>, ptr: *const u8, len: usize) -> Bytes {
let shared = data.load(Ordering::Acquire);
let kind = shared as usize & KIND_MASK;
if kind == KIND_ARC {
shallow_clone_arc(shared as _, ptr, len)
} else {
debug_assert_eq!(kind, KIND_VEC);
shallow_clone_vec(data, shared, shared as *mut u8, ptr, len)
}
}
unsafe fn promotable_odd_drop(data: &mut AtomicPtr<()>, ptr: *const u8, len: usize) {
let shared = *data.get_mut();
let kind = shared as usize & KIND_MASK;
if kind == KIND_ARC {
release_shared(shared as *mut Shared);
} else {
debug_assert_eq!(kind, KIND_VEC);
drop(rebuild_vec(shared as *mut u8, ptr, len));
}
}
unsafe fn rebuild_vec(buf: *mut u8, offset: *const u8, len: usize) -> Vec<u8> {
let cap = (offset as usize - buf as usize) + len;
Vec::from_raw_parts(buf, cap, cap)
}
// ===== impl SharedVtable =====
struct Shared {
@@ -801,44 +872,12 @@ const KIND_MASK: usize = 0b1;
unsafe fn shared_clone(data: &AtomicPtr<()>, ptr: *const u8, len: usize) -> Bytes {
let shared = data.load(Ordering::Acquire);
let kind = shared as usize & KIND_MASK;
if kind == KIND_ARC {
shallow_clone_arc(shared as _, ptr, len)
} else {
debug_assert_eq!(kind, KIND_VEC);
shallow_clone_vec(data, shared, ptr, len)
}
shallow_clone_arc(shared as _, ptr, len)
}
unsafe fn shared_drop(data: &mut AtomicPtr<()>, ptr: *const u8, len: usize) {
unsafe fn shared_drop(data: &mut AtomicPtr<()>, _ptr: *const u8, _len: usize) {
let shared = *data.get_mut();
let kind = shared as usize & KIND_MASK;
if kind == KIND_ARC {
release_shared(shared as *mut Shared);
} else {
debug_assert_eq!(kind, KIND_VEC);
drop(rebuild_vec(shared, ptr, len));
}
}
unsafe fn rebuild_vec(shared: *const (), offset: *const u8, len: usize) -> Vec<u8> {
debug_assert!(
shared as usize & KIND_MASK == KIND_VEC,
"rebuild_vec should have beeen called with KIND_VEC",
);
debug_assert!(
shared as usize & !KIND_MASK != 0,
"rebuild_vec should be called with non-null pointer: {:p}",
shared,
);
let buf = (shared as usize & !KIND_MASK) as *mut u8;
let cap = (offset as usize - buf as usize) + len;
Vec::from_raw_parts(buf, cap, cap)
release_shared(shared as *mut Shared);
}
unsafe fn shallow_clone_arc(shared: *mut Shared, ptr: *const u8, len: usize) -> Bytes {
@@ -857,13 +896,11 @@ unsafe fn shallow_clone_arc(shared: *mut Shared, ptr: *const u8, len: usize) ->
}
#[cold]
unsafe fn shallow_clone_vec(atom: &AtomicPtr<()>, ptr: *const (), offset: *const u8, len: usize) -> Bytes {
unsafe fn shallow_clone_vec(atom: &AtomicPtr<()>, ptr: *const (), buf: *mut u8, offset: *const u8, len: usize) -> Bytes {
// If the buffer is still tracked in a `Vec<u8>`. It is time to
// promote the vec to an `Arc`. This could potentially be called
// concurrently, so some care must be taken.
debug_assert_eq!(ptr as usize & KIND_MASK, KIND_VEC);
// First, allocate a new `Shared` instance containing the
// `Vec` fields. It's important to note that `ptr`, `len`,
// and `cap` cannot be mutated without having `&mut self`.
@@ -871,7 +908,7 @@ unsafe fn shallow_clone_vec(atom: &AtomicPtr<()>, ptr: *const (), offset: *const
// updated and since the buffer hasn't been promoted to an
// `Arc`, those three fields still are the components of the
// vector.
let vec = rebuild_vec(ptr as *const (), offset, len);
let vec = rebuild_vec(buf, offset, len);
let shared = Box::new(Shared {
_vec: vec,
// Initialize refcount to 2. One for this reference, and one