Futex is a fine solution for locks and semaphores (FUTEX_WAIT/WAKE operations). It's been extended repeatedly to handle the needs of condition variables, priority inheritance, timeouts, interop with file descriptors and async/io_uring, etc... with the result that a lot of the API exists to support newer operations with oddball semantics and not a few genuine mistakes and traps (often undocumented). See the glibc condition variable code for how complicated this can get.
Also, while googling for some examples for you I was reminded of this LWN article from a few years back that details some of the issues: https://lwn.net/Articles/823513/
Just because the Linux futex call is currently a Swiss Army knife with some parts that add no value (which I do say in the article) doesn't mean that it's not valuable, or important.
The fact that Linux has extended it in so many ways is, in fact, a testament to it to how impactful the futex concept has been to the world of concurrency.
The fact that it's also at the core of other OS primitives does as well. At least on the MacOS side, those primitives do have much simpler APIs, as well. For instance, here's the main wait function:
The wake side is equally simple, with two calls, one to wake one thread, one to wake all threads. No other number matters, so it's a great simplification in my view.
Your fundamental point is that the futex is actually a pretty unimportant construct. Clearly I don't agree, and it's okay not to agree, but I really am struggling to see your counter-argument.
If futexes aren't important to good locks, then, if modern OSes all felt compelled to jettison the futex for some reason, you'd have pthread implementations do ... what exactly??
ajross|6 months ago
Also, while googling for some examples for you I was reminded of this LWN article from a few years back that details some of the issues: https://lwn.net/Articles/823513/
viega|6 months ago
The fact that Linux has extended it in so many ways is, in fact, a testament to it to how impactful the futex concept has been to the world of concurrency.
The fact that it's also at the core of other OS primitives does as well. At least on the MacOS side, those primitives do have much simpler APIs, as well. For instance, here's the main wait function:
`extern int os_sync_wait_on_address(void * addr, uint64_t value, size_t size, os_sync_wait_on_address_flags_t flags);`
There's also one with a timeout.
The wake side is equally simple, with two calls, one to wake one thread, one to wake all threads. No other number matters, so it's a great simplification in my view.
Your fundamental point is that the futex is actually a pretty unimportant construct. Clearly I don't agree, and it's okay not to agree, but I really am struggling to see your counter-argument.
If futexes aren't important to good locks, then, if modern OSes all felt compelled to jettison the futex for some reason, you'd have pthread implementations do ... what exactly??
senderista|6 months ago
gpderetta|6 months ago