Re: [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions

[Paul E. McKenney]

On Mon, Jul 03, 2017 at 03:49:42PM -0700, Linus Torvalds wrote:
> On Mon, Jul 3, 2017 at 3:30 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > That certainly is one interesting function, isn't it?  I wonder what
> > happens if you replace the raw_spin_is_locked() calls with an
> > unlock under a trylock check?  ;-)
> 
> Deadlock due to interrupts again?

Unless I am missing something subtle, the kgdb_cpu_enter() function in
question has a local_irq_save() over the "interesting" portion of its
workings, so interrupt-handler self-deadlock should not happen.

> Didn't your spin_unlock_wait() patches teach you anything? Checking
> state is fundamentally different from taking the lock. Even a trylock.

That was an embarrassing bug, no two ways about it.  :-/

> I guess you could try with the irqsave versions. But no, we're not doing that.

Again, no need in this case.

But I agree with Will's assessment of this function...

The raw_spin_is_locked() looks to be asking if -any- CPU holds the
dbg_slave_lock, and the answer could of course change immediately
on return from raw_spin_is_locked().  Perhaps the theory is that
if other CPU holds the lock, this CPU is supposed to be subjected to
kgdb_roundup_cpus().  Except that the CPU that held dbg_slave_lock might
be just about to release that lock.  Odd.

Seems like there should be a get_online_cpus() somewhere, but maybe
that constraint is to be manually enforced.

							Thanx, Paul