align multithreading and trap behavior with CM spec - #14146
Merged
dicej merged 10 commits intoAug 31, 2026
Merged
Conversation
dicej
force-pushed
the
spec-threading-and-sync-blocking-updates
branch
3 times, most recently
from
August 17, 2026 17:22
8506000 to
5d88b4c
Compare
alexcrichton
left a comment
alexcrichton
left a comment
Member
There was a problem hiding this comment.
I'd like to review more of concurrent.rs but here's some initial thoughts. It's at the point where whenever I expand context on github it just sends me randomly elsewhere in this diff and I keep losing my spot in the otherwise big diff in concurrent.rs. I'm hoping my changes in dicej#7 which reduce the number of files changed helps with that...
github-actions
Bot
added
the
wasmtime:api
alexcrichton
approved these changes
Aug 19, 2026
Member
|
I think this may also still have lingering management of |
dicej
force-pushed
the
spec-threading-and-sync-blocking-updates
branch
3 times, most recently
from
August 28, 2026 14:09
6a0209e to
25af96d
Compare
dicej
force-pushed
the
spec-threading-and-sync-blocking-updates
branch
2 times, most recently
from
August 28, 2026 19:32
29ac4ee to
b045d7e
Compare
Contributor
Author
|
@alexcrichton would you mind taking one more look at this? Aligning it with WebAssembly/component-model#705 required an overhaul and uncovered a few subtle divergences with the spec which I had to address. |
This updates Wasmtime's Component Model async and cooperative multithreading support to match the current specification, including: - Refined rules for trapping when a sync-typed function blocks. We now enforce this "lazily" rather than "eagerly", mwaning a sync-typed function is allowed to call an async-typed function or blocking intrinsic, and if it doesn't actually block, we won't trap. And if the call _does_ block, we will look for any eligible threads to run and run them until no such threads remain, only trapping if and when we still need to block and have no more threads to run. - Allow reentrance in all cases except when the instance has trapped. - Remove the previous "may block" bookkeeping at the task and root instance level, replacing it with (sub-)instance level tracking of whether any sync-typed function is running in that instance. - Run the event loop during start function calls since they are now allowed to call async-typed functions, create and resume threads, etc. I've also added some code to assert that the event loop is running when it is required. - Add `ConcurrentState::switch_item` for use when we need to run a specific work item at the next turn of the event loop, regardless of what's already in the `high_priority` queue. This is necessary because the spec is particular about which thread to switch to e.g. when calling a function or promoting a thread, and it won't allow us to run any other threads first. Note that this includes `test/component-model` submodule updates which haven't yet been merged to the main branch of the upstream repo, but should be merged soon. See WebAssembly/component-model#705 Fixes bytecodealliance#14117 Co-authored-by: Alex Crichton <alex@alexcrichton.com>
Accept an `unsafe` block which I believe is correct and still otherwise safe at the invocation site. The main hidden constraint now is that we can't transfer fibers to other non-store-bound-locations but that's effectively already true so shouldn't be too onerous to uphold.
- Ensure that subtask status updates are delivered promptly and deterministically according to the spec by using `ConcurrentState::switch_item` instead of `ConcurrentState::high_priority` - Fix reentrance scenarious involving `subtask.cancel` where the subtask tries to add itself to a waitable set before or after being canceled - Refine rules for switching-or-trapping on thread exit when the current instance has a sync-typed call in progress - Misc. bug fixes
Notably, this makes Wasmtime more aggressive about poisoning the store if an error happens when e.g. lifting a result (e.g. due to a misaligned pointer), and the tests have been updated accordingly.
dicej
force-pushed
the
spec-threading-and-sync-blocking-updates
branch
from
August 28, 2026 19:42
b045d7e to
f511826
Compare
dicej
marked this pull request as ready for review
August 28, 2026 19:42
alexcrichton
approved these changes
Aug 28, 2026
github-merge-queue
Bot
removed this pull request from the merge queue due to failed status checks
Aug 31, 2026
Contributor
Author
|
@alexcrichton I think the miri compiler error that kicked it out of the merge queue is a real thing, not a flake; I've reproduced it locally and am trying to address it. |
Member
|
oh huh, when I looked at the failures everything was actually still running so I assumed it was just some github actions flake, but maybe the logs were just delayed? Anyway I'll leave it in your hands in that case |
Contributor
Author
|
Adding |
Member
|
I'd personally prefer to avoid increasing that if we can as I suspect it's going to be a game of whack-a-mole, would it be possible to put a trait object somewhere in the middle perhaps? |
dicej
enabled auto-merge
August 31, 2026 16:42
github-merge-queue
Bot
removed this pull request from the merge queue due to failed status checks
Aug 31, 2026
dicej
enabled auto-merge
August 31, 2026 18:01
github-merge-queue
Bot
removed this pull request from the merge queue due to failed status checks
Aug 31, 2026
dicej
force-pushed
the
spec-threading-and-sync-blocking-updates
branch
from
August 31, 2026 20:06
39b6c8e to
2359e22
Compare
dicej
enabled auto-merge
August 31, 2026 20:07
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
This updates Wasmtime's Component Model async and cooperative multithreading support to match the current specification, including:
Refined rules for trapping when a sync-typed function blocks. We now enforce this "lazily" rather than "eagerly", mwaning a sync-typed function is allowed to call an async-typed function or blocking intrinsic, and if it doesn't actually block, we won't trap. And if the call does block, we will look for any eligible threads to run and run them until no such threads remain, only trapping if and when we still need to block and have no more threads to run.
Ensure that the predicate for determining which threads can be run when a sync-typed function is executing in an instance matches the spec.
Remove the previous "may block" bookkeeping at the task and root instance level, replacing it with (sub-)instance level tracking of whether any sync-typed function is running in that instance.
Run the event loop during start function calls since they are now allowed to call async-typed functions, create and resume threads, etc.
Note that this includes
test/component-modelsubmodule updates which haven't yet been merged to the main branch of the upstream repo, but should be merged soon. See WebAssembly/component-model#696Fixes #14117