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

[Steven Rostedt]

On Sat, 5 Sep 2015 12:30:59 +0200 (CEST)
Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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. 

That was my original solution, and I believe I added patches to do
exactly that to the networking code in the past. I remember writing
that helper function such that on non PREEMPT_RT it was a nop.

I even had that solution in my slides at LinuxCon/LinuxPlumbers ;-)


But then I talk about dcache.c. Take a look at that file, and the
complexity of that. Is it safe to take the inode and dcache parent
locks after you unlock the other locks?

-- Steve
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