chenBright opened a new pull request, #3538:
URL: https://github.com/apache/brpc/pull/3538
### What problem does this PR solve?
Issue Number: resolve #3274
Problem Summary:
The join/end handshake on `version_butex` was not correctly synchronized.
- Producer (`task_runner`, at bthread end): bumped the version with a
plain write
`++*m->version_butex`. The surrounding `version_lock` provides release
semantics
only to threads that also take that lock, but `join()` does not, so
there is no release on
`version_butex` for the join path.
- Consumer (`TaskGroup::join`): exited its wait loop via a plain read
`*m->version_butex`,
with no acquire ordering.
The result is a data race with no happens-before edge, so writes the joined
bthread made
before ending were not guaranteed visible after `join()` returned. On x86
(TSO) the hardware
masked this, but on ARM it surfaced.
PR #3276 attempted a fix by adding a lone `atomic_thread_fence(acquire)`
after the loop. But
per the C++ memory model an acquire fence only establishes synchronization
when a preceding
atomic load reads a value from a matching release operation. Here the load
was a plain read and
the producer had no release store, so the fence pairs with nothing. It
happens to work on ARM
only because the compiler emits a real `dmb ishld`, not because the model
guarantees it.
### What is changed and the side effects?
Changed:
This PR replaces that band-aid with a properly paired release/acquire on
`version_butex`.
Side effects:
- Performance effects:
- Breaking backward compatibility:
---
### Check List:
- Please make sure your changes are compilable.
- When providing us with a new feature, it is best to add related tests.
- Please follow [Contributor Covenant Code of
Conduct](https://github.com/apache/brpc/blob/master/CODE_OF_CONDUCT.md).
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