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Rollup merge of rust-lang#73104 - poliorcetics:explicit-mutex-drop-ex…
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…ample, r=dtolnay

Example about explicit mutex dropping

Fixes rust-lang#67457.

Following the remarks made in rust-lang#73074, I added an example on the main `Mutex` type, with a situation where there is mutable data and a computation result.

In my testing it is effectively needed to explicitly drop the lock, else it deadlocks.

r? @dtolnay because you were the one to review the previous PR.
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Dylan-DPC authored Jun 14, 2020
2 parents f8b99dd + c010e71 commit e1d05a9
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54 changes: 54 additions & 0 deletions src/libstd/sync/mutex.rs
Original file line number Diff line number Diff line change
Expand Up @@ -107,6 +107,60 @@ use crate::sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
///
/// *guard += 1;
/// ```
///
/// It is sometimes necessary to manually drop the mutex guard to unlock it
/// sooner than the end of the enclosing scope.
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// const N: usize = 3;
///
/// let data_mutex = Arc::new(Mutex::new(vec![1, 2, 3, 4]));
/// let res_mutex = Arc::new(Mutex::new(0));
///
/// let mut threads = Vec::with_capacity(N);
/// (0..N).for_each(|_| {
/// let data_mutex_clone = Arc::clone(&data_mutex);
/// let res_mutex_clone = Arc::clone(&res_mutex);
///
/// threads.push(thread::spawn(move || {
/// let mut data = data_mutex_clone.lock().unwrap();
/// // This is the result of some important and long-ish work.
/// let result = data.iter().fold(0, |acc, x| acc + x * 2);
/// data.push(result);
/// drop(data);
/// *res_mutex_clone.lock().unwrap() += result;
/// }));
/// });
///
/// let mut data = data_mutex.lock().unwrap();
/// // This is the result of some important and long-ish work.
/// let result = data.iter().fold(0, |acc, x| acc + x * 2);
/// data.push(result);
/// // We drop the `data` explicitly because it's not necessary anymore and the
/// // thread still has work to do. This allow other threads to start working on
/// // the data immediately, without waiting for the rest of the unrelated work
/// // to be done here.
/// //
/// // It's even more important here than in the threads because we `.join` the
/// // threads after that. If we had not dropped the mutex guard, a thread could
/// // be waiting forever for it, causing a deadlock.
/// drop(data);
/// // Here the mutex guard is not assigned to a variable and so, even if the
/// // scope does not end after this line, the mutex is still released: there is
/// // no deadlock.
/// *res_mutex.lock().unwrap() += result;
///
/// threads.into_iter().for_each(|thread| {
/// thread
/// .join()
/// .expect("The thread creating or execution failed !")
/// });
///
/// assert_eq!(*res_mutex.lock().unwrap(), 800);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "mutex_type")]
pub struct Mutex<T: ?Sized> {
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