X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/9131d1a0288847311e6d15fb8c0cda54d6815d7f..b45ff0e7e99561afa0586c95bf1d6c661e0daf6d:/personal/_posts/2018-07-13-arc-synchronization.md?ds=sidebyside diff --git a/personal/_posts/2018-07-13-arc-synchronization.md b/personal/_posts/2018-07-13-arc-synchronization.md index e2a30c6..033d56e 100644 --- a/personal/_posts/2018-07-13-arc-synchronization.md +++ b/personal/_posts/2018-07-13-arc-synchronization.md @@ -8,7 +8,7 @@ While I was busy doing Rust-unrelated research, [RustBelt](https://plv.mpi-sws.o Notice that I am just the messenger here, the bug was actually found by [Hai](https://people.mpi-sws.org/~haidang/) and [Jacques-Henri](https://jhjourdan.mketjh.fr/). Still, I'd like to use this opportunity to talk a bit about weak memory, synchronization and data races. -This is just a primer, there are tons of resources on the web that go into more detail (for example [here](http://preshing.com/20120913/acquire-and-release-semantics/)). +This is just a primer, there are tons of resources on the web that go into more detail (for example [here](https://preshing.com/20120913/acquire-and-release-semantics/)). @@ -173,7 +173,7 @@ I said that `Mutex`/`RwLock` are good enough *most of the time*. However, `Arc` is one of those cases where the overhead induced by an exclusive lock is just way too big, so it is worth using a more optimized, unsafe implementation. As such, you are going to find plenty of atomic accesses in [the source code of `Arc`](https://github.com/rust-lang/rust/blob/c0955a34bcb17f0b31d7b86522a520ebe7fa93ac/src/liballoc/sync.rs#L201). -And it turns out, as Hai and Jacques-Henri noticed when attempting to prove correctness of [`Arc::get_mut`](https://doc.rust-lang.org/beta/std/sync/struct.Arc.html#method.get_mut), that there is one place where `Relaxed` as used as an ordering, [but it really should have been `Acquire`](https://github.com/rust-lang/rust/pull/52031). +And it turns out, as Hai and Jacques-Henri noticed when attempting to prove correctness of [`Arc::get_mut`](https://doc.rust-lang.org/stable/std/sync/struct.Arc.html#method.get_mut), that there is one place where `Relaxed` as used as an ordering, [but it really should have been `Acquire`](https://github.com/rust-lang/rust/pull/52031). Discussing the exact details of the bug would probably fill another blog post (`Arc` is *really* subtle), but the high-level story is exactly like in our example above: Thanks to `Acquire`, an ordering is induced between the code that follows the `get_mut` and the code in another thread that dropped the last other `Arc`, decrementing the reference count to 1. The PR that fixed the problem contains [some more details in the comments](https://github.com/rust-lang/rust/pull/52031/files). With `Relaxed`, no such ordering is induced, so we have a data race.