Trait frame_support::dispatch::marker::Sync

1.0.0 · source · 
pub unsafe auto trait Sync { }
Expand description

Types for which it is safe to share references between threads.

This trait is automatically implemented when the compiler determines it’s appropriate.

The precise definition is: a type T is Sync if and only if &T is Send. In other words, if there is no possibility of undefined behavior (including data races) when passing &T references between threads.

As one would expect, primitive types like u8 and f64 are all Sync, and so are simple aggregate types containing them, like tuples, structs and enums. More examples of basic Sync types include “immutable” types like &T, and those with simple inherited mutability, such as Box<T>, Vec<T> and most other collection types. (Generic parameters need to be Sync for their container to be Sync.)

A somewhat surprising consequence of the definition is that &mut T is Sync (if T is Sync) even though it seems like that might provide unsynchronized mutation. The trick is that a mutable reference behind a shared reference (that is, & &mut T) becomes read-only, as if it were a & &T. Hence there is no risk of a data race.

Types that are not Sync are those that have “interior mutability” in a non-thread-safe form, such as Cell and RefCell. These types allow for mutation of their contents even through an immutable, shared reference. For example the set method on Cell<T> takes &self, so it requires only a shared reference &Cell<T>. The method performs no synchronization, thus Cell cannot be Sync.

Another example of a non-Sync type is the reference-counting pointer Rc. Given any reference &Rc<T>, you can clone a new Rc<T>, modifying the reference counts in a non-atomic way.

For cases when one does need thread-safe interior mutability, Rust provides atomic data types, as well as explicit locking via sync::Mutex and sync::RwLock. These types ensure that any mutation cannot cause data races, hence the types are Sync. Likewise, sync::Arc provides a thread-safe analogue of Rc.

Any types with interior mutability must also use the cell::UnsafeCell wrapper around the value(s) which can be mutated through a shared reference. Failing to doing this is undefined behavior. For example, transmute-ing from &T to &mut T is invalid.

See the Nomicon for more details about Sync.

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NonNull pointers are not Sync because the data they reference may be aliased.

ThinBox<T> is Sync if T is Sync because the data is owned.

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Bit-Slice Thread Safety

This allows bit-slice references to be moved across thread boundaries only when the underlying T element can tolerate concurrency.

All BitSlice references, shared or exclusive, are only threadsafe if the T element type is Send, because any given bit-slice reference may only have partial control of a memory element that is also being shared by a bit-slice reference on another thread. As such, this is never implemented for Cell<U>, but always implemented for AtomicU and U for a given unsigned integer type U.

Atomic integers safely handle concurrent writes, cells do not allow concurrency at all, so the only missing piece is &mut BitSlice<_, U: Unsigned>. This is handled by the aliasing system that the mutable splitters employ: a mutable reference to an unsynchronized bit-slice can only cross threads when no other handle is able to exist to the elements it governs. Splitting a mutable bit-slice causes the split halves to change over to either atomics or cells, so concurrency is either safe or impossible.

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