Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)

Linus Torvalds
2002-03-08T19:23:29Z

On Fri, 8 Mar 2002, Hubertus Franke wrote:
>
> Could you also comment on the functionality that has been discussed.

First off, I have to say that I really like the current patch by Rusty.
The hashing approach is very clean, and it all seems quite good. As to
specific points:

> (I) the fairness issues that have been raised.
> do you support two wakeup mechanism: FUTEX_UP and FUTEX_UP_FAIR
> or you don't care about fairness and starvation

I don't think fairness and starvation is that big of a deal for
semaphores, usually being unfair in these things tends to just improve
performance through better cache locality with no real downside. That
said, I think the option should be open (which it does seem to be).

For rwlocks, my personal preference is the fifo-fair-preference (unlike
semaphore fairness, I have actually seen loads where read- vs
write-preference really is unacceptable). This might be a point where we
give users the choice.

I do think we should make the lock bigger - I worry that atomic_t simply
won't be enough for things like fair rwlocks, which might want a
"cmpxchg8b" on x86.

So I would suggest making the size (and thus alignment check) of locks at
least 8 bytes (and preferably 16). That makes it slightly harder to put
locks on the stack, but gcc does support stack alignment, even if the code
sucks right now.

Linus