bugfix: update structure One to be thread-safe #175
Merged
Conversation
This file contains 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
basis-library/util/one.sml defines a structure called `One` which
is used in the basis library to optimize memory usage of a few
functions, including:
* Int.{fmt,toString}
* Word.{fmt,toString}
* Real.{split,toManExp}
This patch fixes a buggy race condition in the implementation of
`One`. With this patch, the above functions should be safe for
parallelism and concurrency.
The idea behind `One` is straightforward: a static buffer or mutable
cell is allocated to be shared across calls. When a call is made, if
the shared buffer is not in use, then the shared buffer can be claimed
and used for the duration of that call, and then released.
The mechanism for claiming the buffer (inherited from MLton) was
previously not thread-safe, because it was not atomic at the
hardware level. For MLton, it didn't need to be. But for MPL
this is no longer correct, hence the bug.
This patch switches to using an atomic compare-and-swap (CAS) to
claim the buffer.
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
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Outdated suggestions cannot be applied.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.
basis-library/util/one.sml defines a structure called
Onewhich is used in the basis library to optimize memory usage of a few functions, including:This patch fixes a buggy race condition in the implementation of
One. With this patch, the above functions should be safe for parallelism and concurrency.The idea behind
Oneis straightforward: a static buffer or mutable cell is allocated to be shared across calls. When a call is made, if the shared buffer is not in use, then the shared buffer can be claimed and used for the duration of that call, and then released.The mechanism for claiming the buffer (inherited from MLton) was previously not thread-safe, because it was not atomic at the hardware level. For MLton, it didn't need to be. But for MPL this is no longer correct, hence the bug.
This patch switches to using an atomic compare-and-swap (CAS) to claim the buffer.