[review v2] nptl: Add struct_mutex.h

[Joseph Myers]

On Thu, 14 Nov 2019, Florian Weimer (Code Review) wrote:

> __LOCK_ALIGNMENT should probably removed (replaced by arch-specific
> headers) because it makes the types ABI-incompatible if a non-GNU
> compiler used (missing __attribute__ support).  So we really should
> not encourage its use. But this is separate change.

The list of compiler extensions required to use glibc headers 
(<https://sourceware.org/glibc/wiki/CCompilerFeatures>) is certainly 
incomplete.  Alignment attributes are used in installed headers on various 
architectures for a range of things beyond just pthreads headers.

It's possible it would be possible to use _Alignas / alignas in some cases 
with suitable language standard versions, subject to any issues with 
bounds on supported alignment with those keywords (see e.g. 
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89357>)

> -- 
> Gerrit-Project: glibc
> Gerrit-Branch: master
> Gerrit-Change-Id: I30a22c3e3497805fd6e52994c5925897cffcfe13
> Gerrit-Change-Number: 518
> Gerrit-PatchSet: 2
> Gerrit-Owner: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> Gerrit-Reviewer: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> Gerrit-Reviewer: Florian Weimer <fweimer@redhat.com>
> Gerrit-Comment-Date: Thu, 14 Nov 2019 14:37:04 +0000
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> Gerrit-MessageType: comment

-- 
Joseph S. Myers
joseph@codesourcery.com