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fix potential race in AtomicU64 time monotonizer

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This commit is contained in:
The8472 2021-09-16 18:32:28 +02:00
parent 2b5ddf36fd
commit 2b512cc329
1 changed files with 34 additions and 28 deletions
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62
library/std/src/time/monotonic.rs
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@ -37,35 +37,41 @@ pub mod inner {
// This could be a problem for programs that call instants at intervals greater // This could be a problem for programs that call instants at intervals greater
// than 68 years. Interstellar probes may want to ensure that actually_monotonic() is true. // than 68 years. Interstellar probes may want to ensure that actually_monotonic() is true.
let packed = (secs << 32) | nanos; let packed = (secs << 32) | nanos;
let old = mono.load(Relaxed); let mut old = mono.load(Relaxed);
loop {
if old == UNINITIALIZED || packed.wrapping_sub(old) < u64::MAX / 2 { if old == UNINITIALIZED || packed.wrapping_sub(old) < u64::MAX / 2 {
mono.store(packed, Relaxed); match mono.compare_exchange_weak(old, packed, Relaxed, Relaxed) {
raw Ok(_) => return raw,
} else { Err(x) => {
// Backslide occurred. We reconstruct monotonized time from the upper 32 bit of the old = x;
// passed in value and the 64bits loaded from the atomic continue;
let seconds_lower = old >> 32; }
let mut seconds_upper = secs & 0xffff_ffff_0000_0000; }
if secs & 0xffff_ffff > seconds_lower { } else {
// Backslide caused the lower 32bit of the seconds part to wrap. // Backslide occurred. We reconstruct monotonized time from the upper 32 bit of the
// This must be the case because the seconds part is larger even though // passed in value and the 64bits loaded from the atomic
// we are in the backslide branch, i.e. the seconds count should be smaller or equal. let seconds_lower = old >> 32;
// let mut seconds_upper = secs & 0xffff_ffff_0000_0000;
// We assume that backslides are smaller than 2^32 seconds if secs & 0xffff_ffff > seconds_lower {
// which means we need to add 1 to the upper half to restore it. // Backslide caused the lower 32bit of the seconds part to wrap.
// // This must be the case because the seconds part is larger even though
// Example: // we are in the backslide branch, i.e. the seconds count should be smaller or equal.
// most recent observed time: 0xA1_0000_0000_0000_0000u128 //
// bits stored in AtomicU64: 0x0000_0000_0000_0000u64 // We assume that backslides are smaller than 2^32 seconds
// backslide by 1s // which means we need to add 1 to the upper half to restore it.
// caller time is 0xA0_ffff_ffff_0000_0000u128 //
// -> we can fix up the upper half time by adding 1 << 32 // Example:
seconds_upper = seconds_upper.wrapping_add(0x1_0000_0000); // most recent observed time: 0xA1_0000_0000_0000_0000u128
// bits stored in AtomicU64: 0x0000_0000_0000_0000u64
// backslide by 1s
// caller time is 0xA0_ffff_ffff_0000_0000u128
// -> we can fix up the upper half time by adding 1 << 32
seconds_upper = seconds_upper.wrapping_add(0x1_0000_0000);
}
let secs = seconds_upper | seconds_lower;
let nanos = old as u32;
return ZERO.checked_add_duration(&Duration::new(secs, nanos)).unwrap();
} }
let secs = seconds_upper | seconds_lower;
let nanos = old as u32;
ZERO.checked_add_duration(&Duration::new(secs, nanos)).unwrap()
} }
} }
} }