Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep() hack

[Ingo Molnar <mingo@kernel.org>]

* Thomas Gleixner <tglx@linutronix.de> wrote:

> So the problem we need to solve is:
> 
> retry:
> 	lock(B);
> 	if (!try_lock(A)) {
> 		unlock(B);
> 		cpu_relax();
> 		goto retry;
> 	}
> 
> So instead of doing that proposed magic boost, we can do something
> more straight forward:
> 
> retry:
> 	lock(B);
> 	if (!try_lock(A)) {
> 		lock_and_drop(A, B);
> 		unlock(A);
> 		goto retry;
> 	}
> 
> lock_and_drop() queues the task as a waiter on A, drops B and then
> does the PI adjustment on A. 
> 
> Thoughts?

So why not do:

	lock(B);
	if (!trylock(A)) {
		unlock(B);
		lock(A);
		lock(B);
	}

?

Or, if this can be done, why didn't we do:

	lock(A);
	lock(B);

to begin with?

i.e. I'm not sure the problem is properly specified.

Thanks,

	Ingo