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

Mon May 20 10:10:03 PDT 2019

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.

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.

From: jfbastien at apple.com <jfbastien at apple.com>
Sent: Monday, May 20, 2019 11:27 AM
To: Ben Craig <ben.craig at ni.com>
Cc: Olivier Giroux <OGiroux at nvidia.com>; Mitch Phillips <mitchphillips at outlook.com>; libcxx-dev at lists.llvm.org; kostyak at google.com
Subject: [EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?

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.

On May 20, 2019, at 6:23 AM, Ben Craig <ben.craig at ni.com<mailto:ben.craig at ni.com>> wrote:

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<mailto: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<mailto:jfbastien at apple.com>>; Mitch Phillips <mitchphillips at outlook.com<mailto:mitchphillips at outlook.com>>
Cc: kostyak at google.com<mailto:kostyak at google.com>; libcxx-dev at lists.llvm.org<mailto: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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