wasmtime/runtime/vm/instance/allocator/pooling/
generic_stack_pool.rs

1#![cfg_attr(
2    all(unix, not(miri), not(asan)),
3    expect(dead_code, reason = "not used, but typechecked")
4)]
5
6use crate::PoolConcurrencyLimitError;
7use crate::prelude::*;
8use crate::runtime::vm::PoolingInstanceAllocatorConfig;
9use std::sync::atomic::{AtomicU64, Ordering};
10
11/// A generic implementation of a stack pool.
12///
13/// This implementation technically doesn't actually pool anything at this time.
14/// Originally this was the implementation for non-Unix (e.g. Windows and
15/// MIRI), but nowadays this is also used for fuzzing. For more documentation
16/// for why this is used on fuzzing see the `asan` module in the
17/// `wasmtime-fiber` crate.
18///
19/// Currently the only purpose of `StackPool` is to limit the total number of
20/// concurrent stacks while otherwise leveraging `wasmtime_fiber::FiberStack`
21/// natively.
22#[derive(Debug)]
23pub struct StackPool {
24    stack_size: usize,
25    stack_zeroing: bool,
26    live_stacks: AtomicU64,
27    stack_limit: u64,
28}
29
30impl StackPool {
31    pub fn new(config: &PoolingInstanceAllocatorConfig) -> Result<Self> {
32        Ok(StackPool {
33            stack_size: config.stack_size,
34            stack_zeroing: config.async_stack_zeroing,
35            live_stacks: AtomicU64::new(0),
36            stack_limit: config.limits.total_stacks.into(),
37        })
38    }
39
40    pub fn is_empty(&self) -> bool {
41        self.live_stacks.load(Ordering::Acquire) == 0
42    }
43
44    pub fn allocate(&self) -> Result<wasmtime_fiber::FiberStack> {
45        if self.stack_size == 0 {
46            bail!("fiber stack allocation not supported")
47        }
48
49        let old_count = self.live_stacks.fetch_add(1, Ordering::AcqRel);
50        if old_count >= self.stack_limit {
51            self.live_stacks.fetch_sub(1, Ordering::AcqRel);
52            return Err(PoolConcurrencyLimitError::new(
53                usize::try_from(self.stack_limit).unwrap(),
54                "fibers",
55            )
56            .into());
57        }
58
59        match wasmtime_fiber::FiberStack::new(self.stack_size, self.stack_zeroing) {
60            Ok(stack) => Ok(stack),
61            Err(e) => {
62                self.live_stacks.fetch_sub(1, Ordering::AcqRel);
63                Err(anyhow::Error::from(e))
64            }
65        }
66    }
67
68    pub unsafe fn zero_stack(
69        &self,
70        _stack: &mut wasmtime_fiber::FiberStack,
71        _decommit: impl FnMut(*mut u8, usize),
72    ) {
73        // No need to actually zero the stack, since the stack won't ever be
74        // reused on non-unix systems.
75    }
76
77    /// Safety: see the unix implementation.
78    pub unsafe fn deallocate(&self, stack: wasmtime_fiber::FiberStack) {
79        self.live_stacks.fetch_sub(1, Ordering::AcqRel);
80        // A no-op as we don't actually own the fiber stack on Windows.
81        let _ = stack;
82    }
83}
wasmtime/runtime/vm/instance/allocator/pooling/generic_stack_pool.rs

wasmtime/runtime/vm/instance/allocator/pooling/
generic_stack_pool.rs
