[ajross]

Why is this gray!? This is absolutely correct. Futex was added as an ad hoc solution to the obvious needs of SMP processes communicating via atomic memory operations who still wanted blocking IPC. And it had to be updated and reworked repeatedly as it moved out of the original application (locks and semaphores) into stuff like condition variables and priority inheritance where it didn't work nearly as well.

In point of fact futex is really not a particularly simple syscall and has a lot of traps, see the man page. But the core idea is indeed "not that deep".

[viega]

As the article says, the futex system call is overly complicated. But one shouldn't downplay its importance. Every major OS has had a slimmed down equivalent for about a decade, and the futex is at the core of any good modern lock.

Many things are obvious after, but there was plenty of time before for other people to do the same thing, it's not like we didn't know sysv semaphores didn't scale well.

"ad hoc" feels like an opinion here. My opinion is that when separation of concerns leads to big gains like the futex did, that's elegant, and an achievement. No need to diminish the good work done!

[bicolao]

If this is ad hoc solution, what's the "right" approach?

[ajross]

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/

[viega]

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:

`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??

[gpderetta]

They are so good that they keep being reinvented even in userspace, for example WTF::ParkingLot.

Or C++ also adding std::atomic::wait which is basically a thin [1] wrapper over futex.

[1] implementations still manage to mess it up.

[senderista]

The WTF::ParkingLot example is interesting because it shows that you don't actually need futexes in the kernel to implement them efficiently; you just need something like a spinlock (with sane backoff!) to guard the userspace wait queue and a per-thread eventfd to wake up waiters.

[gpderetta]

Yes, you can do a good futex impression in userspace and add any missing functionality you need. Most importantly for webkit, I think, you get portability.

The advantage of futex provided by the kernel is ABI stability and cross process support.

[saagarjha]

It gives you an implementation. The implementation does not have to be ideal, though.

[senderista]

This is mostly the fault of pthreads, where baroque POSIX semantics had to be shoehorned into the kernel for efficiency.

[gpderetta]

Wait/Wake are enough for the vast majority of the use cases. The rest is a mix of niche cases (PI, robust mutexes) and failed experiments.