forked from M-Labs/libfringe
222 lines
8.6 KiB
Markdown
222 lines
8.6 KiB
Markdown
[![travis][travis-badge]][travis-url]
|
|
[![rustdoc][rustdoc-badge]][rustdoc-url]
|
|
|
|
[travis-badge]: https://img.shields.io/travis/edef1c/libfringe/master.svg?style=flat-square&label=travis
|
|
[travis-url]: https://travis-ci.org/edef1c/libfringe
|
|
[rustdoc-badge]: https://img.shields.io/badge/docs-rustdoc-brightgreen.svg?style=flat-square
|
|
[rustdoc-url]: https://edef1c.github.io/libfringe
|
|
|
|
# libfringe
|
|
|
|
libfringe is a library implementing safe, lightweight context switches,
|
|
without relying on kernel services. It can be used in hosted environments
|
|
(using `std`) as well as on bare metal (using `core`).
|
|
|
|
It provides the following safe abstractions:
|
|
* an implementation of generators,
|
|
[Generator](https://edef1c.github.io/libfringe/fringe/generator/struct.Generator.html).
|
|
|
|
It also provides the necessary low-level building blocks:
|
|
* a trait that can be implemented by stack allocators,
|
|
[Stack](https://edef1c.github.io/libfringe/fringe/struct.Stack.html);
|
|
* a wrapper for using slice references as stacks,
|
|
[SliceStack](https://edef1c.github.io/libfringe/fringe/struct.SliceStack.html);
|
|
* a stack allocator based on `Box<[u8]>`,
|
|
[OwnedStack](https://edef1c.github.io/libfringe/fringe/struct.OwnedStack.html);
|
|
* a stack allocator based on anonymous memory mappings with guard pages,
|
|
[OsStack](https://edef1c.github.io/libfringe/fringe/struct.OsStack.html).
|
|
|
|
libfringe emphasizes safety and correctness, and goes to great lengths to never
|
|
violate the platform ABI.
|
|
|
|
## Usage example
|
|
|
|
```rust
|
|
extern crate fringe;
|
|
|
|
use fringe::{OsStack, Generator};
|
|
|
|
fn main() {
|
|
let stack = OsStack::new(1 << 16).unwrap();
|
|
let mut gen = Generator::new(stack, move |yielder, ()| {
|
|
for i in 1..4 { yielder.generate(i) }
|
|
});
|
|
|
|
println!("{:?}", gen.resume(())); // Some(1)
|
|
println!("{:?}", gen.resume(())); // Some(2)
|
|
println!("{:?}", gen.resume(())); // Some(3)
|
|
println!("{:?}", gen.resume(())); // None
|
|
}
|
|
```
|
|
|
|
## Performance
|
|
|
|
libfringe does context switches in 3ns flat on x86 and x86_64!
|
|
|
|
```
|
|
test swap ... bench: 6 ns/iter (+/- 0)
|
|
```
|
|
|
|
## Debuggability
|
|
|
|
Uniquely among libraries implementing context switching, libfringe ensures that the call stack
|
|
does not abruptly end at the boundary of a generator. Let's consider this buggy code:
|
|
|
|
```rust
|
|
extern crate fringe;
|
|
|
|
use fringe::{OsStack, Generator};
|
|
|
|
fn main() {
|
|
let stack = OsStack::new(1 << 16).unwrap();
|
|
let mut gen = Generator::new(stack, move |yielder, mut index| {
|
|
let values = [1, 2, 3];
|
|
loop { index = yielder.generate(values[index]) }
|
|
});
|
|
|
|
println!("{:?}", gen.resume(5));
|
|
}
|
|
```
|
|
|
|
It crashes with the following backtrace (redacted for clarity):
|
|
|
|
```
|
|
thread 'main' panicked at 'assertion failed: index < self.len()', ../src/libcore/slice.rs:531
|
|
stack backtrace:
|
|
[... core::panicking internals ...]
|
|
9: 0x559ee50f677b - core::panicking::panic::hbfac80217e56ecbe
|
|
10: 0x559ee50b6b4c - core::slice::_<impl core..ops..Index<usize> for [T]>::index::hcb117ddcc7cf2f33
|
|
at .../src/libcore/slice.rs:21
|
|
11: 0x559ee50b7288 - crash_test::main::_{{closure}}::hc7da249d76d51364
|
|
at .../crash_test.rs:9
|
|
12: 0x559ee50b6f23 - _<fringe..generator..Generator<Input, Output, Stack>>::unsafe_new::generator_wrapper::ha2da172d4f041d38
|
|
at .../libfringe/src/generator.rs:94
|
|
13: 0x559ee50b76d3 - fringe::arch::imp::init::trampoline_2::hdb11eb4bdafcdeb9
|
|
at .../libfringe/src/arch/x86_64.rs:71
|
|
14: 0x559ee50b76c4 - fringe::arch::imp::init::trampoline_1::h6b071b2a8ea6aab3
|
|
at .../libfringe/src/arch/x86_64.rs:43
|
|
15: 0x559ee50b7098 - _<fringe..generator..Generator<Input, Output, Stack>>::resume::h8d2b90d386543e29
|
|
at .../libfringe/src/arch/x86_64.rs:131
|
|
at .../libfringe/src/context.rs:52
|
|
at .../libfringe/src/generator.rs:129
|
|
16: 0x559ee50b71c8 - crash_test::main::hfc5e04bc99de7a6a
|
|
at .../crash_test.rs:12
|
|
[... standard library startup internals ...]
|
|
```
|
|
|
|
Similarly, debuggers, profilers, and all other tools using the DWARF debug information have
|
|
full insight into the call stacks.
|
|
|
|
Note that the stack should be deep enough for the panic machinery to store its state—at any point
|
|
there should be at least 8 KiB of free stack space, or panicking will result in a segfault.
|
|
|
|
## Limitations
|
|
|
|
The only architectures currently supported are x86 and x86_64.
|
|
Windows is not supported (see [explanation](#windows-compatibility) below).
|
|
|
|
## Installation
|
|
|
|
libfringe is a [Cargo](https://crates.io) package.
|
|
It's not stable software yet, so you'll have to use it as a git dependency.
|
|
Add this to your `Cargo.toml`:
|
|
|
|
```toml
|
|
[dependencies.fringe]
|
|
git = "https://github.com/edef1c/libfringe.git"
|
|
```
|
|
|
|
To use libfringe on a bare-metal target, add the `no-default-features` key:
|
|
|
|
```toml
|
|
[dependencies.fringe]
|
|
git = "https://github.com/edef1c/libfringe.git"
|
|
no-default-features = true
|
|
```
|
|
|
|
### Feature flags
|
|
|
|
[Cargo's feature flags]: http://doc.crates.io/manifest.html#the-[features]-section
|
|
libfringe provides some optional features through [Cargo's feature flags].
|
|
Currently, all of them are enabled by default.
|
|
|
|
#### `alloc`
|
|
|
|
This flag enables dependency on the `alloc` crate, which is required for
|
|
the [OwnedStack](https://edef1c.github.io/libfringe/fringe/struct.OwnedStack.html).
|
|
|
|
#### `valgrind`
|
|
|
|
This flag enables [Valgrind] integration. libfringe will register context stacks with Valgrind.
|
|
|
|
[Valgrind]: http://valgrind.org
|
|
|
|
## Internals
|
|
|
|
libfringe uses two key implementation techniques.
|
|
|
|
### Compiler-assisted register spilling
|
|
|
|
Traditionally, libraries implementing context switches in userspace have to spill all callee-saved
|
|
registers. On the other hand, libfringe fully inlines calls to every function that eventually
|
|
results in a context switch, and uses an inline assembly statement marking every register as
|
|
clobbered to implement the context switching itself.
|
|
|
|
As a result, only minimal work needs to be performed in the context switching code (LLVM does not
|
|
support spilling the frame pointer), which is especially important on architectures with lots
|
|
of callee-saved registers.
|
|
|
|
### Call stack splicing
|
|
|
|
Non-Windows platforms use [DWARF][] for both stack unwinding and debugging. DWARF call frame
|
|
information is very generic to be ABI-agnostic—it defines a bytecode that describes the actions
|
|
that need to be performed to simulate returning from a function. libfringe uses this bytecode
|
|
to specify that, after the generator function has returned, execution continues at the point
|
|
where the generator function was resumed the last time.
|
|
|
|
[dwarf]: http://dwarfstd.org
|
|
|
|
## Windows compatibility
|
|
|
|
As was said, libfringe emphasizes following the platform ABI. On Windows, the platform ABI
|
|
does not allow moving the stack pointer from the range designated by the OS during thread creation.
|
|
Therefore, the technique used by libfringe on *nix platforms is not applicable, and libfringe
|
|
does not provide Windows support.
|
|
|
|
You might ask, "but what about [mioco][]?" The mioco library uses the [context][] library to
|
|
implement context switches, which is little more than a wrapper of [boost::context][boostcontext].
|
|
The boost::context library changes undocumented fields in the [TIB][] during every context switch
|
|
to try and work around the restrictions placed by the Windows platform ABI. This has
|
|
[failed before][tibfail] and it is bound fail again, breaking existing code that uses
|
|
boost::context in unexpected and complicated ways. The authors of libfringe consider this
|
|
unacceptable.
|
|
|
|
[mioco]: https://github.com/dpc/mioco
|
|
[context]: https://github.com/zonyitoo/context-rs
|
|
[boostcontext]: http://www.boost.org/doc/libs/1_60_0/libs/context/doc/html/context/overview.html
|
|
[TIB]: https://en.wikipedia.org/wiki/Win32_Thread_Information_Block
|
|
[tibfail]: https://svn.boost.org/trac/boost/ticket/8544
|
|
|
|
The only supported way to implement user-mode context switching on Windows is to use [fibers][].
|
|
There are no reasons the safe abstractions provided by libfringe could not be implemented on top
|
|
of that; it is simply not yet done. This should be straightforward and an implementation is
|
|
welcome.
|
|
|
|
[fibers]: https://msdn.microsoft.com/en-us/library/windows/desktop/ms682661(v=vs.85).aspx
|
|
|
|
## License
|
|
|
|
Licensed under either of
|
|
|
|
* Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
|
|
* MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
|
|
|
|
at your option.
|
|
|
|
### Contribution
|
|
|
|
Unless you explicitly state otherwise, any contribution intentionally
|
|
submitted for inclusion in the work by you, as defined in the Apache-2.0
|
|
license, shall be dual licensed as above, without any additional terms or
|
|
conditions.
|