rust/library
Matthias Krüger 5e4eab454a
Rollup merge of #128778 - RalfJung:atomic-read-read-races, r=Mark-Simulacrum
atomics: allow atomic and non-atomic reads to race

We currently define our atomics in terms of C++ `atomic_ref`. That has the unfortunate side-effect of making it UB for an atomic and a non-atomic read to race (concretely, [this code](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d1a743774e60923db33def7fe314d754) has UB). There's really no good reason for this, all the academic models of the C++ memory model I am aware of allow this -- C++ just disallows this because of their insistence on an "object model" with typed memory, where `atomic_ref` temporarily creates an "atomic object" that may not be accesses via regular non-atomic operations.

So instead of tying our operations to `atomic_ref`, let us tie them directly to the underlying C++ memory model. I am not sure what is the best way to phrase this, so here's a first attempt.

We also carve out an exception from the "no mixed-size atomic accesses" rule to permit mixed-size atomic reads -- given that we permit mixed-size non-atomic reads, it seems odd that this would be disallowed for atomic reads. However, when an atomic write races with any other atomic operation, they must use the same size.

With this change, it is finally the case that every non-atomic access can be replaced by an atomic access without introducing UB.

Cc `@rust-lang/opsem` `@chorman0773` `@m-ou-se` `@WaffleLapkin` `@Amanieu`

Fixes https://github.com/rust-lang/unsafe-code-guidelines/issues/483
2024-09-28 15:11:21 +02:00
..
alloc Rollup merge of #130875 - folkertdev:naked-asm-bootstrap, r=tgross35 2024-09-26 22:20:55 -07:00
backtrace@230570f2da Try latest backtrace 2024-08-29 12:13:19 -07:00
core Further clarificarion for atomic and UnsafeCell docs: 2024-09-28 12:14:59 +02:00
panic_abort step cfg(bootstrap) 2024-07-28 14:46:29 -04:00
panic_unwind library: Compute RUST_EXCEPTION_CLASS from native-endian bytes 2024-09-26 11:34:17 -07:00
portable-simd stabilize const_intrinsic_copy 2024-09-23 22:12:54 +02:00
proc_macro Reformat using the new identifier sorting from rustfmt 2024-09-22 19:11:29 -04:00
profiler_builtins Don't skip nonexistent source files 2024-08-27 17:30:42 +10:00
rtstartup Let InstCombine remove Clone shims inside Clone shims 2024-07-25 15:14:42 -04:00
rustc-std-workspace-alloc Replace libstd, libcore, liballoc in line comments. 2022-12-30 14:00:42 +01:00
rustc-std-workspace-core
rustc-std-workspace-std
std Rollup merge of #130922 - tyilo:udp-unspecified, r=ibraheemdev 2024-09-28 09:35:09 +02:00
stdarch@ace72223a0 Fix up standard library intro 2024-09-23 09:21:39 -07:00
sysroot Auto merge of #125016 - nicholasbishop:bishop-cb-112, r=tgross35 2024-07-29 07:41:33 +00:00
test Dogfood feature(file_buffered) 2024-09-24 14:25:16 -07:00
unwind Use &raw in the standard library 2024-09-25 17:03:20 -07:00
windows_targets Win: Add dbghelp to the list of import libraries 2024-09-06 21:21:49 +00:00
Cargo.lock Rollup merge of #130926 - ChrisDenton:cc-1-1-22, r=tgross35 2024-09-27 21:35:09 +02:00
Cargo.toml Auto merge of #129063 - the8472:cold-opt-size, r=Amanieu 2024-09-02 00:58:50 +00:00