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

[Ingo Molnar <mingo@elte.hu>]

* Oleg Nesterov <oleg@tv-sign.ru> wrote:

> > +	spin_lock(&lock->wait_lock);
> > +	__add_waiter(lock, waiter, ti, task __IP__);
> > +	set_task_state(task, task_state);
> 
> I can't understand why __mutex_lock_common() does xchg() after adding 
> the waiter to the ->wait_list. We are holding ->wait_lock, we can't 
> race with __mutex_unlock_nonatomic() - it calls wake_up() and sets 
> ->count under this spinlock.

we must make sure that the drop has not been dropped meanwhile, on the 
way in, between the fastpath-unlock atomic op, and the xchg() here.

	Ingo