<div dir="ltr">I'm a little confused about how said aformentioned freestanding implementation would work. We (GWP-ASan + Scudo) can't depend on having libcxx (we actually <a href="https://reviews.llvm.org/D62048">explicitly disable</a> using of the c++ standard library for these libraries), and I don't know where we could have a shared implementation live (that could be used by both us and libcxx).<div><br></div><div>Would we have a spinlock implementation in libcxx and have that guarded by a macro definition before the include of <mutex>, and then keep a synchronised version in compiler-rt?</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, May 20, 2019 at 10:10 AM Ben Craig <<a href="mailto:ben.craig@ni.com">ben.craig@ni.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal">Wouldn’t all the Scudo and sanitizer things count as part of the toolchain implementation though? Having pieces of the toolchain use an alternative name seems pretty reasonable to me.<u></u><u></u></p>
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<p class="MsoNormal">If the goal is to have a freestanding std::mutex that is implemented as a spin lock, well, that’s different. I could be convinced that’s a good thing to have, but I’m still a bit uneasy about it.<u></u><u></u></p>
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<p class="MsoNormal"><b>From:</b> <a href="mailto:jfbastien@apple.com" target="_blank">jfbastien@apple.com</a> <<a href="mailto:jfbastien@apple.com" target="_blank">jfbastien@apple.com</a>> <br>
<b>Sent:</b> Monday, May 20, 2019 11:27 AM<br>
<b>To:</b> Ben Craig <<a href="mailto:ben.craig@ni.com" target="_blank">ben.craig@ni.com</a>><br>
<b>Cc:</b> Olivier Giroux <<a href="mailto:OGiroux@nvidia.com" target="_blank">OGiroux@nvidia.com</a>>; Mitch Phillips <<a href="mailto:mitchphillips@outlook.com" target="_blank">mitchphillips@outlook.com</a>>; <a href="mailto:libcxx-dev@lists.llvm.org" target="_blank">libcxx-dev@lists.llvm.org</a>; <a href="mailto:kostyak@google.com" target="_blank">kostyak@google.com</a><br>
<b>Subject:</b> [EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?<u></u><u></u></p>
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<p class="MsoNormal">I think what developers *use* should definitely be named std::mutex. We indeed want to consider how the ABI is exposed, that’s a good question for libc++ maintainers.<u></u><u></u></p>
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<p class="MsoNormal">On May 20, 2019, at 6:23 AM, Ben Craig <<a href="mailto:ben.craig@ni.com" target="_blank">ben.craig@ni.com</a>> wrote:<u></u><u></u></p>
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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.<u></u><u></u></p>
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<p class="MsoNormal"><b>From:</b><span class="gmail-m_-1501418984181503871apple-converted-space"> </span>libcxx-dev <<a href="mailto:libcxx-dev-bounces@lists.llvm.org" target="_blank">libcxx-dev-bounces@lists.llvm.org</a>><span class="gmail-m_-1501418984181503871apple-converted-space"> </span><b>On Behalf Of<span class="gmail-m_-1501418984181503871apple-converted-space"> </span></b>Olivier
Giroux via libcxx-dev<br>
<b>Sent:</b><span class="gmail-m_-1501418984181503871apple-converted-space"> </span>Friday, May 17, 2019 7:55 PM<br>
<b>To:</b><span class="gmail-m_-1501418984181503871apple-converted-space"> </span>JF Bastien <<a href="mailto:jfbastien@apple.com" target="_blank">jfbastien@apple.com</a>>; Mitch Phillips <<a href="mailto:mitchphillips@outlook.com" target="_blank">mitchphillips@outlook.com</a>><br>
<b>Cc:</b><span class="gmail-m_-1501418984181503871apple-converted-space"> </span><a href="mailto:kostyak@google.com" target="_blank">kostyak@google.com</a>;
<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank">libcxx-dev@lists.llvm.org</a><br>
<b>Subject:</b><span class="gmail-m_-1501418984181503871apple-converted-space"> </span>[EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?<u></u><u></u></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.<u></u><u></u></p>
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<p class="MsoNormal">Olivier<u></u><u></u></p>
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<p class="MsoNormal"><b><span style="font-size:12pt">From:<span class="gmail-m_-1501418984181503871apple-converted-space"> </span></span></b><span style="font-size:12pt">libcxx-dev <<a href="mailto:libcxx-dev-bounces@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev-bounces@lists.llvm.org</span></a>>
on behalf of JF Bastien via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>><br>
<b>Reply-To:<span class="gmail-m_-1501418984181503871apple-converted-space"> </span></b>JF Bastien <<a href="mailto:jfbastien@apple.com" target="_blank"><span style="color:purple">jfbastien@apple.com</span></a>><br>
<b>Date:<span class="gmail-m_-1501418984181503871apple-converted-space"> </span></b>Friday, May 17, 2019 at 4:40 PM<br>
<b>To:<span class="gmail-m_-1501418984181503871apple-converted-space"> </span></b>Mitch Phillips <<a href="mailto:mitchphillips@outlook.com" target="_blank"><span style="color:purple">mitchphillips@outlook.com</span></a>><br>
<b>Cc:<span class="gmail-m_-1501418984181503871apple-converted-space"> </span></b>"<a href="mailto:kostyak@google.com" target="_blank"><span style="color:purple">kostyak@google.com</span></a>" <<a href="mailto:kostyak@google.com" target="_blank"><span style="color:purple">kostyak@google.com</span></a>>, "<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>"
<<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>><br>
<b>Subject:<span class="gmail-m_-1501418984181503871apple-converted-space"> </span></b>Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?</span><u></u><u></u></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.<u></u><u></u></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>?<u></u><u></u></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" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">Hi all,<span class="gmail-m_-1501418984181503871apple-converted-space"> </span><u></u><u></u></p>
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<p class="MsoNormal">In a recent<span class="gmail-m_-1501418984181503871apple-converted-space"> </span><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=" target="_blank"><span style="color:purple">discussion</span></a><span class="gmail-m_-1501418984181503871apple-converted-space"> </span>from
the reviews of<span class="gmail-m_-1501418984181503871apple-converted-space"> </span><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=" target="_blank"><span style="color:purple">GWP-ASan</span></a>,
it was mentioned that I should probably consult with cxx-dev to see whether they'd be interested in a common spinlock implementation.<u></u><u></u></p>
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<p class="MsoNormal">The problem is:<u></u><u></u></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.<u></u><u></u></p>
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<p class="MsoNormal"> - GWP-ASan (as it's packaged into Scudo) also requires a spinlock.<u></u><u></u></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.<u></u><u></u></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?<u></u><u></u></p>
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<p class="MsoNormal">Cheers,<u></u><u></u></p>
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<p class="MsoNormal">Mitch.<u></u><u></u></p>
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<p class="MsoNormal">_______________________________________________<br>
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