Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

[Darren Hart <dvhltc@us.ibm.com>]

drepper@gmail.com wrote:
> On Tue, Apr 6, 2010 at 16:16, Thomas Gleixner <tglx@linutronix.de> wrote:
>> I know that you can do any weird stuff with the futex value, but I
>> don't see the "dramatic" limitation. Care to elaborate ?
> 
> If we have to fill in the PID we can represent only three states in a 
> futex: 0, PID, -PID.  Today we can represent 2^32 states.  Quite a 
> difference.

For general futexes sure, but not for robust or PI mutexes. Having 
adaptive futexes be based on the TID|WAITERS_FLAG policy certainly isn't 
breaking new ground, and is consistent with the other kernel-side futex 
locking implementations.

However, I agree that a FUTEX_SET_WAIT_ADAPTIVE sort of call might be 
very powerful. However, that might be just academic until I can show an 
improvement with adaptive futexes.

>> The per thread pinned page would be unconditional, right ?
> 
> Only if the process would be using these adaptive mutexes.  It could be 
> conditional.

What about the concern of this TLS approach only working for process 
private locks? I would very much like to make this work for both shared 
and private locks.

Thanks,

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team