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in our memory model, all atomics are supposed to be full barriers; stores are not release-only. this is important because store is used as an unlock operation in places where it needs to acquire the waiter count to determine if a futex wake is needed. at least in the malloc-internal locks, but possibly elsewhere, soft deadlocks from missing futex wake (breakable by poking the threads to restart the syscall, e.g. by attaching a tracer) were reported to occur. once the malloc lock is replaced with Jens Gustedt's new lock implementation (see commit 47d0bcd4762f223364e5b58d5a381aaa0cbd7c38), malloc will not be affected by the issue, but it's not clear that other uses won't be. reducing the strength of the ordering properties required from a_store would require a thorough analysis of how it's used. to fix the problem, I'm removing the powerpc[64]-specific a_store definition; now, the top-level atomic.h will implement a_store using a_barrier on both sides of the store. it's not clear to me yet whether there might be issues with the other atomics. it's possible that a_post_llsc needs to be replaced with a full barrier to guarantee the formal semanics we want, but either way I think the difference is unlikely to impact the way we use them.
musl libc
musl, pronounced like the word "mussel", is an MIT-licensed
implementation of the standard C library targetting the Linux syscall
API, suitable for use in a wide range of deployment environments. musl
offers efficient static and dynamic linking support, lightweight code
and low runtime overhead, strong fail-safe guarantees under correct
usage, and correctness in the sense of standards conformance and
safety. musl is built on the principle that these goals are best
achieved through simple code that is easy to understand and maintain.
The 1.1 release series for musl features coverage for all interfaces
defined in ISO C99 and POSIX 2008 base, along with a number of
non-standardized interfaces for compatibility with Linux, BSD, and
glibc functionality.
For basic installation instructions, see the included INSTALL file.
Information on full musl-targeted compiler toolchains, system
bootstrapping, and Linux distributions built on musl can be found on
the project website:
http://www.musl-libc.org/