test/contention: release size store first #184
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
mjp41
requested changes
May 13, 2020
| @@ -86,7 +86,7 @@ void test_tasks_f(size_t id) | |||
|
|
|||
| *res = size; | |||
| size_t* out = | |||
| contention[n % swapsize].exchange(res, std::memory_order_relaxed); | |||
| contention[n % swapsize].exchange(res, std::memory_order_release); | |||
There was a problem hiding this comment.
I think you are right, but shouldn't it be std::memory_order_acq_rel as this performs as a read as well, that needs to induce the happens-before edge?
There was a problem hiding this comment.
Yea, I think you're right. I think I thought the data dependencies in this thread would be sufficient, but they're not.
It is important, in test_tasks_f, that the store of the size to the allocated block be made visible to other processors before the store of the pointer itself. Otherwise, other cores are justified in reading junk. This manifests on PowerPC as tripping the "Deallocating with incorrect size supplied" assertion in alloc.h:/check_size because the value read from the allocated block may not be a size but rather an internal queue pointer, which is implausibly large, as sizes go.
mjp41
approved these changes
May 13, 2020
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Outdated suggestions cannot be applied.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.
It is important, in test_tasks_f, that the store of the size to the
allocated block be made visible to other processors before the store of
the pointer itself. Otherwise, other cores are justified in reading
junk.
This manifests on PowerPC as tripping the "Deallocating with incorrect
size supplied" assertion in alloc.h:/check_size because the value read
from the allocated block may not be a size but rather an internal queue
pointer, which is implausibly large, as sizes go.