Re: [patch 05/15] Generic Mutex Subsystem, mutex-core.patch

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> How expensive is the xchg?  Since __mutex_lock_common is called even 
> when it's going to wake up. Maybe it might be more efficient to add 
> something like:
> 
>           if (atomic_cmpxchg(&lock->count, 1, 0) {
>               debug_set_owner(lock, ti __IP__);
>               debug_unlock_irqrestore(&debug_lock, *flags, ti);
>               return 1;
> 	  }
> 
> This way we save the overhead of grabbing another spinlock, adding the 
> task to the wait_list and changing it's state.

in the first pass we definitely need to add ourselves to the list first 
- hence have to grab the lock. Even after the schedule(), we have to 
xchg it to -1, not 0. This is crutial to 'not drop the ball' property of 
one-waiter-in-flight logic - we must not lose the -1 'there are more 
waiters pending' property. Plus, we have the grab the lock because we 
remove ourselves from the wait-list after the schedule(). So i'm not 
sure your suggested optimization is possible.

	Ingo