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

Mon May 20 14:05:30 PDT 2019

> On May 20, 2019, at 2:03 PM, Ben Craig <ben.craig at ni.com> wrote:
> 
> I guess it depends if the goal is to have a spin lock class, or a sharded lock table.
>  
> If it’s a spin lock class, then someone needs to just make the class and get it into some header and/or static lib somewhere.  The specifics aren’t super important.

The former.

> If it’s a sharded lock table, then you need to figure out which dynamic library will provide it, and how that will interact with libc++ when it is used atop libsupc++ or built with GCC.
>  
> From: jfbastien at apple.com <jfbastien at apple.com> 
> Sent: Monday, May 20, 2019 3:59 PM
> To: Mitch Phillips <mitchphillips at outlook.com>
> Cc: Ben Craig <ben.craig at ni.com>; Olivier Giroux <OGiroux at nvidia.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?
>  
> “Freestanding” is meant for a few things, of which: codebases that can’t have external dependencies, “bare metal” code or other embedded code. This sounds exactly like what you’re after.
>  
> The projects are all in the same repository. You can absolutely depend on having it: it’s in the same repo. You therefore don’t need to duplicate anything.
>  
> 
> 
> On May 20, 2019, at 1:55 PM, Mitch Phillips <mitchphillips at outlook.com <mailto:mitchphillips at outlook.com>> wrote:
>  
> 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 explicitly disable <https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D62048&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=0JDRKupnKG2FyAYi69CfJqswn5ys3q3D8nrjL8eqtQw&s=AgRXbJ1EHKQtYIn_0Ku-RB1v_LizN2Q4nVeZ595pZa8&e=> 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).
>  
> 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?
>  
> On Mon, May 20, 2019 at 10:10 AM Ben Craig <ben.craig at ni.com <mailto:ben.craig at ni.com>> wrote:
> 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 <mailto:jfbastien at apple.com> <jfbastien at apple.com <mailto:jfbastien at apple.com>> 
> Sent: Monday, May 20, 2019 11:27 AM
> To: Ben Craig <ben.craig at ni.com <mailto:ben.craig at ni.com>>
> Cc: Olivier Giroux <OGiroux at nvidia.com <mailto:OGiroux at nvidia.com>>; Mitch Phillips <mitchphillips at outlook.com <mailto:mitchphillips at outlook.com>>; libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>;kostyak at google.com <mailto: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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