pointers are complicated: clarify that this post is not about exploring the trade-off

[web.git] / ralf / _posts / 2018-07-13-arc-synchronization.md

diff --git a/ralf/_posts/2018-07-13-arc-synchronization.md b/ralf/_posts/2018-07-13-arc-synchronization.md
index e2a30c60d2566453aa7157e40fdfe6cabbdb203a..7579dcf46a17b802966eeea4d2ce6836c805cb77 100644 (file)
--- a/ralf/_posts/2018-07-13-arc-synchronization.md
+++ b/ralf/_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/)).
 
 <!-- MORE -->
 
@@ -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` was 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.
@@ -190,3 +190,5 @@ We were realistic enough to find [another bug]({% post_url 2017-06-09-mutexguard
 Hai and Jacques-Henri are currently working on remedying this particular simplification by extending the first RustBelt paper to also cover weak memory, and that's when they ran into this problem.
 
 **Update:** Turns out Servo has a [copy of `Arc`](https://doc.servo.org/servo_arc/index.html) that [has the same problem](https://github.com/servo/servo/issues/21186). So we got two bugs for the price of one. :)  **/Update**
+
+#### Footnotes