Re: [PATCH PREEMPT RT] rt-mutex: fix deadlock in device mapper

[Thomas Gleixner]

On Tue, 21 Nov 2017, Mike Galbraith wrote:
> On Mon, 2017-11-20 at 16:33 -0500, Mikulas Patocka wrote:
> > 
> > Is there some specific scenario where you need to call 
> > blk_schedule_flush_plug from rt_spin_lock_fastlock?
> 
> Excellent question.  What's the difference between not getting IO
> started because you meet a mutex with an rt_mutex under the hood, and
> not getting IO started because you meet a spinlock with an rt_mutex
> under the hood?  If just doing the mutex side puts this thing back to
> sleep, I'm happy.

Think about it from the mainline POV.

The spinlock cannot ever go to schedule and therefore cannot create a
situation which requires an unplug. The RT substitution of the spinlock
with a rtmutex based sleeping spinlock should not change that at all.

A regular mutex/rwsem etc. can and will unplug when the lock is contended
and the caller blocks. The RT conversion of these locks to rtmutex based
variants creates the problem: Unplug cannot be called when the task has
pi_blocked_on set because the unplug path might content on yet another
lock. So unplugging in the slow path before setting pi_blocked_on is the
right thing to do.

Thanks,

	tglx