Update part01.rs

[rust-101.git] / src / part15.rs

diff --git a/src/part15.rs b/src/part15.rs
index f60a481bfc7a3302f9bfe6f04f0695ab88746db5..e22d507c615328be99e338ff892001640bffdd28 100644 (file)
--- a/src/part15.rs
+++ b/src/part15.rs
@@ -5,29 +5,35 @@ use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
 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`
 //@ 
 //@ ## `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.
+//@ 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`.
+//@ 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.
+//@ 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, 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<Mutex<usize>>);
 
 #[derive(Clone)]
 struct ConcurrentCounter(Arc<Mutex<usize>>);
 


@@ -39,21 +45,25 @@ impl ConcurrentCounter {
 
     // The core operation is, of course, `increment`.
     pub fn increment(&self, by: usize) {
 
     // 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.
         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.
     }
 
     // The function `get` returns the current value of the counter.


@@ -91,58 +101,74 @@ pub fn main() {
         println!("Current value: {}", counter.get());
     }
 
         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());
 }
 
     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_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.
+// **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`
 
 //@ ## `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`
 
 //@ ## `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.
 //@ 
 //@ 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<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.
+//@ 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 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`.
+//@ 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) | [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)