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Switch to using a single atomic and treating 0 as 'uninitialized'

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This commit is contained in:
Thom Chiovoloni 2020-10-08 17:03:16 -07:00
parent e4cf24bd45
commit 59c06e9e40
1 changed files with 12 additions and 16 deletions
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28
library/std/src/sys/unix/time.rs
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@ -117,7 +117,7 @@ impl Hash for Timespec {
#[cfg(any(target_os = "macos", target_os = "ios"))] #[cfg(any(target_os = "macos", target_os = "ios"))]
mod inner { mod inner {
use crate::fmt; use crate::fmt;
use crate::sync::atomic::{AtomicBool, AtomicU64, Ordering}; use crate::sync::atomic::{AtomicU64, Ordering};
use crate::sys::cvt; use crate::sys::cvt;
use crate::sys_common::mul_div_u64; use crate::sys_common::mul_div_u64;
use crate::time::Duration; use crate::time::Duration;
@ -232,15 +232,19 @@ mod inner {
} }
fn info() -> mach_timebase_info { fn info() -> mach_timebase_info {
static INITIALIZED: AtomicBool = AtomicBool::new(false); // INFO_BITS conceptually is an `Option<mach_timebase_info>`. We can do
// this in 64 bits because we know 0 is never a valid value for the
// `denom` field.
//
// Encoding this as a single `AtomicU64` allows us to use `Relaxed`
// operations, as we are only interested in in the effects on a single
// memory location.
static INFO_BITS: AtomicU64 = AtomicU64::new(0); static INFO_BITS: AtomicU64 = AtomicU64::new(0);
// If a previous thread has initialized `INFO_BITS`, use that. // If a previous thread has initialized `INFO_BITS`, use it.
if INITIALIZED.load(Ordering::Acquire) { let info_bits = INFO_BITS.load(Ordering::Relaxed);
// Note: `Relaxed` is correct here and below -- the `Acquire` / if info_bits != 0 {
// `Release` pair used for `INITIALIZED` ensures this load can see return info_from_bits(info_bits);
// the corresponding store below.
return info_from_bits(INFO_BITS.load(Ordering::Relaxed));
} }
// ... otherwise learn for ourselves ... // ... otherwise learn for ourselves ...
@ -252,15 +256,7 @@ mod inner {
unsafe { unsafe {
mach_timebase_info(&mut info); mach_timebase_info(&mut info);
} }
// This is racy, but the race should be against other threads trying to
// write the same value.
INFO_BITS.store(info_to_bits(info), Ordering::Relaxed); INFO_BITS.store(info_to_bits(info), Ordering::Relaxed);
// The `Release` here "publishes" the store of `INFO_BITS` to other
// threads (which do a `INITIALIZED.load(Acquire)`) despite it being
// read/written w/ `Relaxed`.
INITIALIZED.store(true, Ordering::Release);
info info
} }