[libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?

[Ben Craig via libcxx-dev]

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.

From: libcxx-dev <libcxx-dev-bounces at lists.llvm.org> On Behalf Of Olivier Giroux via libcxx-dev
Sent: Friday, May 17, 2019 7:55 PM
To: JF Bastien <jfbastien at apple.com>; Mitch Phillips <mitchphillips at outlook.com>
Cc: kostyak at google.com; libcxx-dev at lists.llvm.org
Subject: [EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?

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.

Olivier

From: libcxx-dev <libcxx-dev-bounces at lists.llvm.org<mailto:libcxx-dev-bounces at lists.llvm.org>> on behalf of JF Bastien via libcxx-dev <libcxx-dev at lists.llvm.org<mailto:libcxx-dev at lists.llvm.org>>
Reply-To: JF Bastien <jfbastien at apple.com<mailto:jfbastien at apple.com>>
Date: Friday, May 17, 2019 at 4:40 PM
To: Mitch Phillips <mitchphillips at outlook.com<mailto:mitchphillips at outlook.com>>
Cc: "kostyak at google.com<mailto:kostyak at google.com>" <kostyak at google.com<mailto:kostyak at google.com>>, "libcxx-dev at lists.llvm.org<mailto:libcxx-dev at lists.llvm.org>" <libcxx-dev at lists.llvm.org<mailto:libcxx-dev at lists.llvm.org>>
Subject: Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?

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.

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>?


On May 17, 2019, at 1:49 PM, Mitch Phillips via libcxx-dev <libcxx-dev at lists.llvm.org<mailto:libcxx-dev at lists.llvm.org>> wrote:

Hi all,

In a recent discussion<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=> from the reviews of GWP-ASan<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=>, it was mentioned that I should probably consult with cxx-dev to see whether they'd be interested in a common spinlock implementation.

The problem is:
 - Scudo hardened allocator (compiler-rt/lib/scudo) requires its own spinlock as it can't use the C++ standard library due to Fuchsia requirements.
 - GWP-ASan (as it's packaged into Scudo) also requires a spinlock.
 - 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.

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?

Cheers,
Mitch.
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