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0 pointsjavert13y ago0 comments

I'm not certain, but I guess pthread mutexes probably use userspace mutexes under the covers.

Correct. man 7 pthreads states:

In NPTL, thread synchronization primitives (mutexes, thread joining, and so on) are implemented using the Linux futex(2) system call.

A futex is a mutex that only does a system call if there is contention; you can google it.

0 comments

scott_s13y ago

"Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux": http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf

robertgraham13y ago

My blog post was quite clear that I'm talking about futexes, and then when it fails to get a lock that it goes into the kernel. Seriously, that's how everyone did it from Solaris to Windows before Linux "invented" the concept AND it's been in Linux for a decade. When somebody says "mutex", you have to assume they already mean "futex".

javertOP13y ago

If you fail to get the lock, then you have to wait on the lock, so you might as well go into the kernel anyway. That's the reason futexes do that, instead of just letting the thread spin.

True, there are scenarios when you might prefer to spin, but they're pretty specialized.

My blog post was quite clear that I'm talking about futexes

No, you never mention the word "futex."

When somebody says "mutex", you have to assume they already mean "futex".

I don't think that's a valid generalization at all.

robertgraham13y ago

The entire point of the post was talk about "lock-free" algorithms where two cores can make forward progress without either having to wait or spin.

What systems have mutexes that aren't built like futexes?

1 more reply

jamesaguilar13y ago

> I don't think that's a valid generalization at all.

Especially given he was complaining about syscall costs . . . which are nonexistent with futexes . . .

1 more reply

jamesaguilar13y ago

If there is contention, I think you mean, right?

javertOP13y ago

Yeah, edited to fix it. Just a typo.

j / k navigate · click thread line to collapse

0 pointsjavert13y ago0 comments

I'm not certain, but I guess pthread mutexes probably use userspace mutexes under the covers.

Correct. man 7 pthreads states:

In NPTL, thread synchronization primitives (mutexes, thread joining, and so on) are implemented using the Linux futex(2) system call.

A futex is a mutex that only does a system call if there is contention; you can google it.

0 comments

scott_s13y ago

"Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux": http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf

robertgraham13y ago

javertOP13y ago

If you fail to get the lock, then you have to wait on the lock, so you might as well go into the kernel anyway. That's the reason futexes do that, instead of just letting the thread spin.

True, there are scenarios when you might prefer to spin, but they're pretty specialized.

My blog post was quite clear that I'm talking about futexes

No, you never mention the word "futex."

When somebody says "mutex", you have to assume they already mean "futex".

I don't think that's a valid generalization at all.

robertgraham13y ago

The entire point of the post was talk about "lock-free" algorithms where two cores can make forward progress without either having to wait or spin.

What systems have mutexes that aren't built like futexes?

1 more reply

jamesaguilar13y ago

> I don't think that's a valid generalization at all.

Especially given he was complaining about syscall costs . . . which are nonexistent with futexes . . .