[comex]

> On the other hand, I have yet to learn whether Mac OS has any synchronisation primitives that mitigate mutex priority inversion.

It does! xnu supports priority inheritance through "turnstiles", a kernel-internal mechanism which is used by default by a number of locking primitives (list at [1]), including normal pthread mutexes (though not read-write locks [2]), as well as the os_unfair_lock API (via the ulock syscalls). With pthread mutexes, you can actually explicitly request priority inheritance by calling pthread_mutexattr_setprotocol [3] with PTHREAD_PRIO_INHERIT; the Apple implementation supports it, but currently ignores the protocol setting and just gives all mutexes priority inheritance.

[1] https://opensource.apple.com/source/xnu/xnu-4903.241.1/osfmk...

[2] https://opensource.apple.com/source/libpthread/libpthread-33..., search for _kwq_use_turnstile

[2] https://linux.die.net/man/3/pthread_mutexattr_setprotocol

[RossBencina]

> xnu supports priority inheritance through "turnstiles"

Do you have any information on how long this "turnstiles" feature has been available? [1] I was aware that the userspace pthread libraries implement but ignore PTHREAD_PRIO_INHERIT, but wasn't aware that the underlying implementation gave priority inheritance semantics.

Update: Looks like the "turnstiles" feature may have its origins in Solaris. [2]

[1] This link would suggest as of Mac OS 10.14: http://newosxbook.com/articles/12-10.14.html

[2] http://sunsite.uakom.sk/sunworldonline/swol-08-1999/swol-08-...

[comex]

Yeah, looks like 10.14 and iOS 12. I actually hadn't realized it was so new. Before that, it seems that priority inheritance ("promotion") had long existed for kernel-internal locks [1], but not for anything in userland. It also seems that the behavior of supporting but ignoring PTHREAD_PRIO_INHERIT long predates turnstiles, which is pretty annoying.

As for origins... xnu's implementation appears to be written from scratch, but in terms of inspiration for the design and name, I'm not sure. Aside from Solaris, another possibility is FreeBSD [2] (but note that xnu's turnstiles are in the Mach side of the codebase rather than the BSD side).

[1] https://opensource.apple.com/source/xnu/xnu-1504.15.3/osfmk/...

[2] https://github.com/freebsd/freebsd/commits/master/sys/sys/tu...