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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Change std::call_once to use per-flag lock rather than global lock"
href="https://bugs.llvm.org/show_bug.cgi?id=44152">44152</a>
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<th>Summary</th>
<td>Change std::call_once to use per-flag lock rather than global lock
</td>
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<th>Product</th>
<td>libc++
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<th>Version</th>
<td>unspecified
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>dehuan@motorola.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
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<pre>In libc++, std::call_once uses a global lock. This seems like a poor design
choice.
Code here:
<a href="https://github.com/llvm/llvm-project/blob/master/libcxx/src/mutex.cpp#L226">https://github.com/llvm/llvm-project/blob/master/libcxx/src/mutex.cpp#L226</a>
Previously MSVC also used global lock but it was changed in 2015:
<a href="https://devblogs.microsoft.com/cppblog/stl-fixes-in-vs-2015-part-2/">https://devblogs.microsoft.com/cppblog/stl-fixes-in-vs-2015-part-2/</a>
More story:
<a href="https://stackoverflow.com/questions/22692783/is-stdcall-once-reentrant-and-thread-safe">https://stackoverflow.com/questions/22692783/is-stdcall-once-reentrant-and-thread-safe</a>
<a href="https://stackoverflow.com/questions/26477070/concurrent-stdcall-once-calls">https://stackoverflow.com/questions/26477070/concurrent-stdcall-once-calls</a>
For scalability, I believe there should be one mutex per std::once_flag.</pre>
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