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Attempt at fixing mpmc_bounded_queue on 32bit where integer overflow is a concern. #10258

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Attempt at fixing mpmc_bounded_queue on 32bit where integer overflow is a concern. #10258

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22 changes: 15 additions & 7 deletions src/libstd/rt/mpmc_bounded_queue.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -34,7 +34,7 @@ use option::*;
use vec;
use clone::Clone;
use kinds::Send;
use num::{Exponential,Algebraic,Round};
use uint;

struct Node<T> {
sequence: AtomicUint,
Expand Down Expand Up @@ -62,8 +62,7 @@ impl<T: Send> State<T> {
if capacity < 2 {
2u
} else {
// use next power of 2 as capacity
2f64.pow(&((capacity as f64).log2().ceil())) as uint
uint::next_power_of_two(capacity)
}
} else {
capacity
Expand All @@ -84,12 +83,13 @@ impl<T: Send> State<T> {
}

fn push(&mut self, value: T) -> bool {
let buffer_len = self.buffer.len();
let mask = self.mask;
let mut pos = self.enqueue_pos.load(Relaxed);
loop {
let node = &mut self.buffer[pos & mask];
let seq = node.sequence.load(Acquire);
let diff: int = seq as int - pos as int;
let diff = seq - pos;

if diff == 0 {
let enqueue_pos = self.enqueue_pos.compare_and_swap(pos, pos+1, Relaxed);
Expand All @@ -100,7 +100,11 @@ impl<T: Send> State<T> {
} else {
pos = enqueue_pos;
}
} else if (diff < 0) {
} else if diff < 0 {
return false
} else if pos == 0 && (seq-1) as uint % buffer_len == 0 {
// handle the case where enqueue_pos has overflowed
// back to 0 but the queue is full
return false
} else {
pos = self.enqueue_pos.load(Relaxed);
Expand All @@ -115,8 +119,12 @@ impl<T: Send> State<T> {
loop {
let node = &mut self.buffer[pos & mask];
let seq = node.sequence.load(Acquire);
let diff: int = seq as int - (pos + 1) as int;
if diff == 0 {
let diff: i64 = seq as i64 - (pos as i64 + 1i64);
if diff == 0 || (seq == 0 && pos == uint::max_value) {
// the part after || handles the case where
// pos+1 would overflow back to 0 causing
// diff to be negative and thus dequeue to fail
// when there is infact data in the queue
let dequeue_pos = self.dequeue_pos.compare_and_swap(pos, pos+1, Relaxed);
if dequeue_pos == pos {
let value = node.value.take();
Expand Down