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Fix flaky tests caused by relaxed load in C++ 'shared_ptr::use_count()' #450

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merged 2 commits into from
Sep 2, 2020
Merged

Fix flaky tests caused by relaxed load in C++ 'shared_ptr::use_count()' #450

merged 2 commits into from
Sep 2, 2020

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@piscisaureus piscisaureus commented Aug 31, 2020 •
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piscisaureus added a commit to piscisaureus/safe_v8 that referenced this pull request Aug 31, 2020
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Fix minor SharedPtr/SharedRef issues, and add unit tests (denoland#450)
ef8dec2 
* The `Default` trait did not actually get derived for `SharedPtr<T>`.
  This is solved by implementing `Default` manually.
* Trait function `Shared::get()` used to return a mutable raw pointer
  (`*mut Self`), but it would be inconceivable to ever mutate the
  referenced value. It was changed to return a const pointer instead.
* Added some basic unit tests for types `SharedPtr` and `SharedRef`.
piscisaureus added a commit to piscisaureus/safe_v8 that referenced this pull request Aug 31, 2020
@piscisaureus
Fix flakyness caused by C++ shared_ptr::use_count() relaxed load (d…
8c86eb8 
…enoland#450)

Fixes: denoland#284
@piscisaureus piscisaureus changed the title X Fix flaky tests due to C++ shared_ptr::use_count() relaxed load Aug 31, 2020
@piscisaureus piscisaureus changed the title Fix flaky tests due to C++ shared_ptr::use_count() relaxed load Fix flaky tests due to C++ 'shared_ptr::use_count()' using relaxed load Aug 31, 2020
piscisaureus added a commit to piscisaureus/safe_v8 that referenced this pull request Aug 31, 2020
@piscisaureus
Fix flaky tests due to C++ 'shared_ptr::use_count()' using relaxed lo…
ec0c5d4 
…ad (denoland#450)

Fixes: denoland#284
piscisaureus added a commit to piscisaureus/safe_v8 that referenced this pull request Aug 31, 2020
@piscisaureus
Fix flaky tests caused by relaxed load in C++ 'shared_ptr::use_count(…
4b31cbb 
…)' (denoland#450)

Fixes: denoland#284
@piscisaureus piscisaureus changed the title Fix flaky tests due to C++ 'shared_ptr::use_count()' using relaxed load Fix flaky tests caused by relaxed load in C++ 'shared_ptr::use_count()' Aug 31, 2020
@piscisaureus piscisaureus requested a review from ry August 31, 2020 06:29
@piscisaureus piscisaureus reopened this Aug 31, 2020
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ry commented Aug 31, 2020

Are we even using shared_ptr in a multithreaded way? I don't really understand what is happening here.

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LGTM - but consider removing assert_use_count_eq from the public API

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piscisaureus commented Sep 2, 2020 •
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Are we even using shared_ptr in a multithreaded way? I don't really understand what is happening here.

Theoretically the thread could be preempted, or a page fault could happen, in between dropping the shared_ptr reference and the call to use_count(), and then the thread resumes on a different core. However when reproducing this locally it happens often enough (about 1 in 100 times) that I don't believe this is happening.

My alternative theory is that this is what SPECTRE mitigation looks like.

@bnoordhuis I wonder if you have any theories...

@piscisaureus
Fix minor SharedPtr/SharedRef issues, and add unit tests (denoland#450)
d91970d 
* The `Default` trait did not actually get derived for `SharedPtr<T>`.
  This is solved by implementing `Default` manually.
* Trait function `Shared::get()` used to return a mutable raw pointer
  (`*mut Self`), but it would be inconceivable to ever mutate the
  referenced value. It was changed to return a const pointer instead.
* Added some basic unit tests for types `SharedPtr` and `SharedRef`.
@piscisaureus
Fix flaky tests caused by relaxed load in C++ 'shared_ptr::use_count(…
21f6ecf 
…)' (denoland#450)

Fixes: denoland#284
@piscisaureus piscisaureus merged commit 21f6ecf into denoland:master Sep 2, 2020
@piscisaureus piscisaureus deleted the shared branch September 2, 2020 23:51
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