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when the kernel is responsible for waking waiters on a robust mutex whose owner died, it does not have a waiters count available and must rely entirely on the waiter bit of the lock value. normally, this bit is only set by newly arriving waiters, so it will be clear if no new waiters arrived after the current owner obtained the lock, even if there are other waiters present. leaving it clear is desirable because it allows timed-lock operations to remove themselves as waiters and avoid causing unnecessary futex wake syscalls. however, for process-shared robust mutexes, we need to set the bit whenever there are existing waiters so that the kernel will know to wake them. for non-process-shared robust mutexes, the wake happens in userspace and can look at the waiters count, so the bit does not need to be set in the non-process-shared case.
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/