diff options
| -rw-r--r-- | library/std/src/sys/unix/time.rs | 28 |
1 files changed, 12 insertions, 16 deletions
diff --git a/library/std/src/sys/unix/time.rs b/library/std/src/sys/unix/time.rs index d6a5cc26df2..fac4b05ad0b 100644 --- a/library/std/src/sys/unix/time.rs +++ b/library/std/src/sys/unix/time.rs @@ -117,7 +117,7 @@ impl Hash for Timespec { #[cfg(any(target_os = "macos", target_os = "ios"))] mod inner { use crate::fmt; - use crate::sync::atomic::{AtomicBool, AtomicU64, Ordering}; + use crate::sync::atomic::{AtomicU64, Ordering}; use crate::sys::cvt; use crate::sys_common::mul_div_u64; use crate::time::Duration; @@ -232,15 +232,19 @@ mod inner { } 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); - // If a previous thread has initialized `INFO_BITS`, use that. - if INITIALIZED.load(Ordering::Acquire) { - // Note: `Relaxed` is correct here and below -- the `Acquire` / - // `Release` pair used for `INITIALIZED` ensures this load can see - // the corresponding store below. - return info_from_bits(INFO_BITS.load(Ordering::Relaxed)); + // If a previous thread has initialized `INFO_BITS`, use it. + let info_bits = INFO_BITS.load(Ordering::Relaxed); + if info_bits != 0 { + return info_from_bits(info_bits); } // ... otherwise learn for ourselves ... @@ -252,15 +256,7 @@ mod inner { unsafe { 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); - - // 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 } |