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

[Avi Kivity]

On 04/06/2010 07:14 PM, Thomas Gleixner wrote:
> > IMO the best solution is to spin in userspace while the lock holder is
> > running, fall into the kernel when it is scheduled out.
> >      
> That's just not realistic as user space has no idea whether the lock
> holder is running or not and when it's scheduled out without a syscall :)
>    

The kernel could easily expose this information by writing into the 
thread's TLS area.

So:

- the kernel maintains a current_cpu field in a thread's tls
- lock() atomically writes a pointer to the current thread's current_cpu 
when acquiring
- the kernel writes an invalid value to current_cpu when switching out
- a contended lock() retrieves the current_cpu pointer, and spins as 
long as it is a valid cpu

--
error compiling committee.c: too many arguments to function

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