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Delete historical interruptable support in Wasmtime (#3925)

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* Delete historical interruptable support in Wasmtime

This commit removes the `Config::interruptable` configuration along with
the `InterruptHandle` type from the `wasmtime` crate. The original
support for adding interruption to WebAssembly was added pretty early on
in the history of Wasmtime when there was no other method to prevent an
infinite loop from the host. Nowadays, however, there are alternative
methods for interruption such as fuel or epoch-based interruption.

One of the major downsides of `Config::interruptable` is that even when
it's not enabled it forces an atomic swap to happen when entering
WebAssembly code. This technically could be a non-atomic swap if the
configuration option isn't enabled but that produces even more branch-y
code on entry into WebAssembly which is already something we try to
optimize. Calling into WebAssembly is on the order of a dozens of
nanoseconds at this time and an atomic swap, even uncontended, can add
up to 5ns on some platforms.

The main goal of this PR is to remove this atomic swap on entry into
WebAssembly. This is done by removing the `Config::interruptable` field
entirely, moving all existing consumers to epochs instead which are
suitable for the same purposes. This means that the stack overflow check
is no longer entangled with the interruption check and perhaps one day
we could continue to optimize that further as well.

Some consequences of this change are:

* Epochs are now the only method of remote-thread interruption.
* There are no more Wasmtime traps that produces the `Interrupted` trap
  code, although we may wish to move future traps to this so I left it
  in place.
* The C API support for interrupt handles was also removed and bindings
  for epoch methods were added.
* Function-entry checks for interruption are a tiny bit less efficient
  since one check is performed for the stack limit and a second is
  performed for the epoch as opposed to the `Config::interruptable`
  style of bundling the stack limit and the interrupt check in one. It's
  expected though that this is likely to not really be measurable.
* The old `VMInterrupts` structure is renamed to `VMRuntimeLimits`.
This commit is contained in:
Alex Crichton
2022-03-14 15:25:11 -05:00
committed by GitHub
parent 62a6a7ab6c
commit c22033bf93
33 changed files with 293 additions and 589 deletions
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9
crates/fuzzing/src/oracles.rs
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@@ -107,9 +107,6 @@ pub enum Timeout {
/// No timeout is used, it should be guaranteed via some other means that
/// the input does not infinite loop.
None,
/// A time-based timeout is used with a sleeping thread sending a signal
/// after the specified duration.
Time(Duration),
/// Fuel-based timeouts are used where the specified fuel is all that the
/// provided wasm module is allowed to consume.
Fuel(u64),
@@ -143,12 +140,6 @@ pub fn instantiate(wasm: &[u8], known_valid: bool, config: &generators::Config,
// This prevents us from creating a huge number of sleeping threads if
// this function is executed in a loop, like it does on nightly fuzzing
// infrastructure.
Timeout::Time(timeout) => {
let handle = store.interrupt_handle().unwrap();
timeout_state.spawn_timeout(timeout, move || handle.interrupt());
}
// Similar to above, but we bump the epoch rather than set the
// interrupt flag.
Timeout::Epoch(timeout) => {
let engine = store.engine().clone();
timeout_state.spawn_timeout(timeout, move || engine.increment_epoch());