On Sat, 5 Sep 2015 08:18:36 -0400
Steven Rostedt [off-list ref] wrote:

On Sat, 5 Sep 2015 12:30:59 +0200 (CEST)
Thomas Gleixner [off-list ref] wrote:

quoted

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.

Just to point out again that I misread what you wrote. That's what I
get for responding to email 10 minutes after I get out of bed ;-)


You need to be careful about adding the waiter on A. If the owner of A
is blocked on B, the pi inheritance may detect that as a deadlock.

-- Steve