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<p class="MsoNormal">I think it’s fine to give such a class the same interface as std::mutex, just please don’t actually name it std::mutex. I would be pretty concerned about ABI compatibility in that situation, as well as subtle interactions with things like
condition_variable::wait. cxxabi::__mutex, or almost any other identifier would be fine.<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> libcxx-dev <libcxx-dev-bounces@lists.llvm.org> <b>
On Behalf Of </b>Olivier Giroux via libcxx-dev<br>
<b>Sent:</b> Friday, May 17, 2019 7:55 PM<br>
<b>To:</b> JF Bastien <jfbastien@apple.com>; Mitch Phillips <mitchphillips@outlook.com><br>
<b>Cc:</b> kostyak@google.com; libcxx-dev@lists.llvm.org<br>
<b>Subject:</b> [EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?<o:p></o:p></p>
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<p class="MsoNormal">That’s what makes the most sense to me, include <mutex> with a macro that freestanding would also use. Inside of that, I would have an implementation of standard mutex backed by atomics and the atomic_wait/atomic_notify_* functions, themselves
configured with a macro to not rely on the OS in freestanding implementations.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Olivier<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">libcxx-dev <<a href="mailto:libcxx-dev-bounces@lists.llvm.org">libcxx-dev-bounces@lists.llvm.org</a>> on behalf of JF Bastien via
libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>><br>
<b>Reply-To: </b>JF Bastien <<a href="mailto:jfbastien@apple.com">jfbastien@apple.com</a>><br>
<b>Date: </b>Friday, May 17, 2019 at 4:40 PM<br>
<b>To: </b>Mitch Phillips <<a href="mailto:mitchphillips@outlook.com">mitchphillips@outlook.com</a>><br>
<b>Cc: </b>"<a href="mailto:kostyak@google.com">kostyak@google.com</a>" <<a href="mailto:kostyak@google.com">kostyak@google.com</a>>, "<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>" <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>><br>
<b>Subject: </b>Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?<o:p></o:p></span></p>
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<p class="MsoNormal">I think it makes sense for libc++ to have a version of mutex which has the same API as the standard one, but for “freestanding” platforms such as yours. A flavor which mostly just spins and yields as in your review.
<o:p></o:p></p>
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<p class="MsoNormal">I’m not sure how to best turn it on, though. Should it be controlled by a macro, and otherwise it just looks like you’re using <mutex>?<o:p></o:p></p>
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<p class="MsoNormal">On May 17, 2019, at 1:49 PM, Mitch Phillips via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hi all, <o:p></o:p></p>
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<p class="MsoNormal">In a recent <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D61923-231503272&d=DwMGaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=yw9dJtPZSvMNi00nrRSUlnN4NTQzKvXzHbd5w8ShH8A&s=uW10gtroz30NxGIUU3BvNH5efGnzv2tds3P49eY5ri4&e=">
discussion</a> from the reviews of <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D60593&d=DwMGaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=yw9dJtPZSvMNi00nrRSUlnN4NTQzKvXzHbd5w8ShH8A&s=rKVj5vHVGEsEZRjMkiN2X6e0bAfdTY7iVW0laX2AO8o&e=">
GWP-ASan</a>, it was mentioned that I should probably consult with cxx-dev to see whether they'd be interested in a common spinlock implementation.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The problem is:<o:p></o:p></p>
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<p class="MsoNormal"> - Scudo hardened allocator (compiler-rt/lib/scudo) requires its own spinlock as it can't use the C++ standard library due to Fuchsia requirements.<o:p></o:p></p>
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<p class="MsoNormal"> - GWP-ASan (as it's packaged into Scudo) also requires a spinlock.<o:p></o:p></p>
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<p class="MsoNormal"> - Compiler-rt sanitizer_common also can't use c++ stdlib, and has its own spinlock. We can't reuse the sanitizer_common spinlock for Scudo/GWP-ASan as it's currently tightly coupled into sanitizer_common, and Scudo+Fuchsia can't afford
the code size overhead of pulling the entire sanitizer_common library.<o:p></o:p></p>
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<p class="MsoNormal">The plan was to basically write a small standalone spinlock implementation that can be used by all three of these requirements. Would libcxx benefit by us making this an llvm-common spinlock rather than compiler-rt-common? If so, are there
any requirements that we need to be aware of?<o:p></o:p></p>
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<p class="MsoNormal">Cheers,<o:p></o:p></p>
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<p class="MsoNormal">Mitch.<o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
libcxx-dev mailing list<br>
<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a><br>
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