-// Rust-101, Part 15: Interior Mutability (cont.), RefCell, Cell, Drop
+// Rust-101, Part 15: Mutex, Interior Mutability (cont.), RwLock, Sync
// ===================================================================
-//@ [`RefCell`](http://doc.rust-lang.org/beta/std/cell/struct.RefCell.html)
-//@ [`is very much like `RwLock`, but it's not thread-safe: "Locking" is done without atomic operations.
-//@ One can also see it as a dynamically checked version of Rust's usual borrowing rules. You have to explicitly say
-//@ when you want to borrow the data in there shared, or mutably, and Rust will complain at run-time if you have
-//@ a mutable borrow while any other borrow is active. You can then write programs that Rust may otherwise not
-//@ accept. Sending a shared borrow to this to another thread is dangerous, as the checks are not performed in
-//@ a thread-safe manner. However, sending the *entire* `RefCell` is okay, because there's only ever one owner, and all
-//@ we need to ensure is that everybody attempting to borrow is in the same thread as the owner. <br/>
-//@ [`Cell<T>`](http://doc.rust-lang.org/beta/std/cell/struct.Cell.html) is like a stripped-down version of `RefCell<T>`: It doesn't allow
-//@ you to borrow its content. Instead, it has a methods `get` and `set` to change the value stored in the cell, and to copy it out.
-//@ For obvious reasons, this requires `T` to be `Copy`.
-//@
-//@ You can also think about all these types coming from the other end: Starting with `Cell`, we have a primitive for
-//@ interior mutability that provides `get` and `set`, both just requiring a shared borrow. Think of these functions as
-//@ mutating the *content* of the cell, but not the cell itself, the container. (Just like in ML, where assignment to a
-//@ `ref` changes the content, not the location.) However, due to the ownership discipline, `Cell` only works for types
-//@ that are `Copy`. Hence we also have `RefCell`, which allows working with the data right in the cell, rather than
-//@ having to copy it out. `RefCell` uses non-atomic operations for this purpose, so for the multi-threaded setting, there's
-//@ the thread-safe `RwLock`. And finally, in case a distinction between readers and writers is not helpful, one can use the
-//@ more efficient `Mutex`.
-
-//@ [index](main.html) | [previous](part14.html) | [next](main.html)
+use std::sync::{Arc, Mutex};
+use std::thread;
+
+//@ We already saw that we can use `Arc` to share memory between threads. However, `Arc` can only provide *read-only*
+//@ access to memory: Since there is aliasing, Rust cannot, in general, permit mutation. To implement shared-memory
+//@ concurrency, we need to have aliasing and permutation - following, of course, some strict rules to make sure
+//@ 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.
+//@
+//@ ## `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
+//@ becomes possible because of the discipline of ownership and borrowing.
+//@
+//@ As an example, let us write a concurrent counter. As usual in Rust, we first have to think about our data layout:
+//@ That will be `Mutex<usize>`. Of course, we want multiple threads to have access to this `Mutex`, so we wrap it in an `Arc`.
+//@
+//@ Rather than giving every field a name, a struct can also be defined by just giving a sequence of types (similar
+//@ to how a variant of an `enum` is defined). This is called a *tuple struct*. It is often used when constructing
+//@ a *newtype*, as we do here: `ConcurrentCounter` is essentially just a new name for `Arc<Mutex<usize>>`. However,
+//@ is is a locally declared types, so we can give it an inherent implementation and implement traits for it. Since the
+//@ field is private, nobody outside this module can even know the type we are wrapping.
+
+// The derived `Clone` implementation will clone the `Arc`, so all clones will actually talk about the same counter.
+#[derive(Clone)]
+struct ConcurrentCounter(Arc<Mutex<usize>>);
+
+impl ConcurrentCounter {
+ // The constructor just wraps the constructors of `Arc` and `Mutex`.
+ pub fn new(val: usize) -> Self {
+ ConcurrentCounter(Arc::new(Mutex::new(val))) /*@*/
+ }
+
+ // The core operation is, of course, `increment`.
+ pub fn increment(&self, by: usize) {
+ // `lock` on a mutex returns a guard, very much like `RefCell`. The guard gives access to the data contained in the mutex.
+ //@ (We will discuss the `unwrap` soon.) `.0` is how we access the first component of a tuple or a struct.
+ let mut counter = self.0.lock().unwrap();
+ //@ The guard is a smart pointer to the content.
+ *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
+ //@ 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!
+ //@
+ //@ One of the subtle aspects of locking is *poisoning*. If a thread panics while it holds a lock, it could leave the
+ //@ data-structure in a bad state. The lock is hence considered *poisoned*. Future attempts to `lock` it will fail.
+ //@ Above, we simply assert via `unwrap` that this will never happen. Alternatively, we could have a look at the poisoned
+ //@ state and attempt to recover from it.
+ }
+
+ // The function `get` returns the current value of the counter.
+ pub fn get(&self) -> usize {
+ let counter = self.0.lock().unwrap(); /*@*/
+ *counter /*@*/
+ }
+}
+
+// Now our counter is ready for action.
+pub fn main() {
+ let counter = ConcurrentCounter::new(0);
+
+ // We clone the counter for the first thread, which increments it by 2 every 15ms.
+ let counter1 = counter.clone();
+ let handle1 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep_ms(15);
+ counter1.increment(2);
+ }
+ });
+
+ // The second thread increments the counter by 3 every 20ms.
+ let counter2 = counter.clone();
+ let handle2 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep_ms(20);
+ counter2.increment(3);
+ }
+ });
+
+ // Now we watch the threads working on the counter.
+ for _ in 0..50 {
+ thread::sleep_ms(5);
+ println!("Current value: {}", counter.get());
+ }
+
+ // Finally, we wait for all the threads to finish to be sure we can catch the counter's final value.
+ handle1.join().unwrap();
+ handle2.join().unwrap();
+ println!("Final value: {}", counter.get());
+}
+
+// **Exercise 15.1**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
+// `by` *only if* the current value is `test`.
+//
+// **Exercise 15.2**: Rather than panicking in case the lock is poisoned, we can use `into_innter` on the error to recover
+// 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`](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
+//@ 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`
+//@ 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`](https://doc.rust-lang.org/stable/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
+//@ 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.
+//@ 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`.
+//@ 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`.
+//@
+//@ 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](part16.html)
projects / rust-101.git / blobdiff