Re: [PATCH 1/3] rcu: Use static initializer for krc.lock

[Uladzislau Rezki <urezki@gmail.com>]

> On 2020-04-16 17:38:44 [+0200], Uladzislau Rezki wrote:
> > > What is the worst thing that can happen if a CPU migration happens
> > > between raw_cpu_ptr() and spin_lock_irqsave() ?
> > > 
> > You will just introduce higher rate of lock contention what we would
> > like to get rid of. 
> 
> The possibility of the migration is minimal. So it is not _always_
> happening.
> 
Agree, it is minimal. But if possible the possibility should be 0 :)

> >                     Could you please explain what is broken when you
> > do like:
> > 
> > <snip>
> > local_irq_save(flags);
> > krcp = this_cpu_ptr(&krc);
> > spin_lock(&krcp->lock);
> > <snip>
> 
> As written in my previous email to Joel (who somehow fell of the Cc
> list in this part of the thread):
> |local_irq_save() + spin_lock() is the problem, see
> |    https://www.kernel.org/doc/html/latest/locking/locktypes.html#spinlock-t-and-rwlock-t
> 
You mean that in CONFIG_PREEMPT_RT:

<snip>
 local_irq_save(flags);
 krcp = this_cpu_ptr(&krc);
 spin_lock(&krcp->lock);
<snip>

it will not be preempted, what does not follow PREEMPT_RT policy. Is that correct?

--
Vlad Rezki