//@ there are no data races. In Rust, shared-memory concurrency is obtained through *interior mutability*,
//@ which we already discussed in a single-threaded context in part 12.
//@
-//@ The most basic type for interior mutability that supports concurrency is [`Mutex<T>`](http://doc.rust-lang.org/stable/std/sync/struct.Mutex.html).
+//@ ## `Mutex`
+//@ The most basic type for interior mutability that supports concurrency is [`Mutex<T>`](https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html).
//@ This type implements *critical sections* (or *locks*), but in a data-driven way: One has to specify
//@ the type of the data that's protected by the mutex, and Rust ensures that the data is *only* accessed
//@ through the mutex. In other words, "lock data, not code" is actually enforced by the type system, which
*counter = *counter + by;
//@ At the end of the function, `counter` is dropped and the mutex is available again.
//@ This can only happen when full ownership of the guard is given up. In particular, it is impossible for us
- //@ to borrow some of its content, release the lock of the mutex, and subsequently access the protected data without holding
+ //@ to take a reference to some of its content, release the lock of the mutex, and subsequently access the protected data without holding
//@ the lock. Enforcing the locking discipline is expressible in the Rust type system, so we don't have to worry
//@ about data races *even though* we are mutating shared memory!
//@
// the data inside the lock. Change the code above to do that. Try using `unwrap_or_else` for this job.
//@ ## `RwLock`
-//@ Besides `Mutex`, there's also [`RwLock`](http://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which
+//@ Besides `Mutex`, there's also [`RwLock`](https://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which
//@ provides two ways of locking: One that grants only read-only access, to any number of concurrent readers, and another one
-//@ for exclusive write access. Notice that this is the same pattern we already saw with shared vs. mutable borrows. Hence
+//@ for exclusive write access. Notice that this is the same pattern we already saw with shared vs. mutable references. Hence
//@ another way of explaining `RwLock` is to say that it is like `RefCell`, but works even for concurrent access. Rather than
//@ panicking when the data is already borrowed, `RwLock` will of course block the current thread until the lock is available.
//@ In this view, `Mutex` is a stripped-down version of `RwLock` that does not distinguish readers and writers.
// **Exercise 15.3**: Change the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
-//@ ## Sync
+//@ ## `Sync`
//@ Clearly, if we had used `RefCell` rather than `Mutex`, the code above could not work: `RefCell` is not prepared for
//@ multiple threads trying to access the data at the same time. How does Rust make sure that we don't accidentally use
//@ `RefCell` across multiple threads?
//@
//@ In part 13, we talked about types that are marked `Send` and thus can be moved to another thread. However, we did *not*
-//@ talk about the question whether a borrow is `Send`. For `&mut T`, the answer is: It is `Send` whenever `T` is send.
-//@ `&mut` allows moving values back and forth, it is even possible to [`swap`](http://doc.rust-lang.org/beta/std/mem/fn.swap.html)
-//@ the contents of two mutably borrowed values. So in terms of concurrency, sending a mutable borrow is very much like
+//@ talk about the question whether a reference is `Send`. For `&mut T`, the answer is: It is `Send` whenever `T` is send.
+//@ `&mut` allows moving values back and forth, it is even possible to [`swap`](https://doc.rust-lang.org/stable/std/mem/fn.swap.html)
+//@ the contents of two mutable references. So in terms of concurrency, sending a mutable, unique reference is very much like
//@ sending full ownership, in the sense that it can be used to move the object to another thread.
//@
-//@ But what about `&T`, a shared borrow? Without interior mutability, it would always be all-right to send such values.
+//@ But what about `&T`, a shared reference? Without interior mutability, it would always be all-right to send such values.
//@ After all, no mutation can be performed, so there can be as many threads accessing the data as we like. In the
//@ presence of interior mutability though, the story gets more complicated. Rust introduces another marker trait for
//@ this purpose: `Sync`. A type `T` is `Sync` if and only if `&T` is `Send`. Just like `Send`, `Sync` has a default implementation
//@ and is thus automatically implemented for a data-structure *if* all its members implement it.
//@
-//@ Since `Arc` provides multiple threads with a shared borrow of its content, `Arc<T>` is only `Send` if `T` is `Sync`.
+//@ Since `Arc` provides multiple threads with a shared reference to its content, `Arc<T>` is only `Send` if `T` is `Sync`.
//@ So if we had used `RefCell` above, which is *not* `Sync`, Rust would have caught that mistake. Notice however that
//@ `RefCell` *is* `Send`: If ownership of the entire cell is moved to another thread, it is still not possible for several
//@ threads to try to access the data at the same time.
//@
-//@ Almost all the types we saw so far are `Sync`, with the exception of `Rc`. Remember that a shared borrow is good enough
-//@ for cloning, and we don't want other threads to clone our local `Rc`, so it must not be `Sync`. The rule of `Mutex`
-//@ is to enforce synchronization, so it should not be entirely surprising that `Mutex<T>` is `Send` *and* `Sync` provided that
-//@ `T` is `Send`.
+//@ Almost all the types we saw so far are `Sync`, with the exception of `Rc`. Remember that a shared reference is good enough
+//@ for cloning, and we don't want other threads to clone our local `Rc` (they would race for updating the reference count),
+//@ so it must not be `Sync`. The rule of `Mutex` is to enforce synchronization, so it should not be entirely surprising that
+//@ `Mutex<T>` is `Send` *and* `Sync` provided that `T` is `Send`.
//@
//@ You may be curious whether there is a type that's `Sync`, but not `Send`. There are indeed rather esoteric examples
//@ of such types, but that's not a topic I want to go into. In case you are curious, there's a
//@ [Rust RFC](https://github.com/rust-lang/rfcs/blob/master/text/0458-send-improvements.md), which contains a type `RcMut` that would be `Sync` and not `Send`.
//@ You may also be interested in [this blog post](https://huonw.github.io/blog/2015/02/some-notes-on-send-and-sync/) on the topic.
-//@ [index](main.html) | [previous](part14.html) | [next](main.html)
+//@ [index](main.html) | [previous](part14.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part15.rs) | [next](part16.html)
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