X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/2d40516a8393db1f27bb822ff95c71a1a9c82537..refs/heads/master:/src/part15.rs diff --git a/src/part15.rs b/src/part15.rs index 6c99f4d..e22d507 100644 --- a/src/part15.rs +++ b/src/part15.rs @@ -5,29 +5,35 @@ use std::sync::{Arc, Mutex}; use std::thread; use std::time::Duration; -//@ 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. +//@ 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`](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. +//@ The most basic type for interior mutability that supports concurrency is +//@ [`Mutex`](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`. Of course, we want multiple threads to have access to this `Mutex`, so we wrap it in an `Arc`. +//@ 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`. 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>`. 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. +//@ 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>`. However, it 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>); @@ -39,21 +45,25 @@ impl ConcurrentCounter { // 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. + // `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 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! + //@ This can only happen when full ownership of the guard is given up. In particular, it is + //@ impossible for us 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! //@ - //@ 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. + //@ 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. @@ -91,58 +101,74 @@ pub fn main() { 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. + // 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 15.1**: 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_inner` 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. +// **Exercise 15.2**: Rather than panicking in case the lock is poisoned, we can use `into_inner` +// 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 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. +//@ 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 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` -//@ 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? +//@ 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 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 +//@ 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 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 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. +//@ 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 reference to its content, `Arc` 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. +//@ Since `Arc` provides multiple threads with a shared reference to its content, `Arc` 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 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` 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` +//@ 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) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part15.rs) | [next](part16.html) +//@ 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) | [raw source](workspace/src/part15.rs) | +//@ [next](part16.html)