Re: [Qestion] Is preempt_disable/enable needed in non-preemption code path

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Apr 16, 2021 at 12:18:42AM +0800, Xu, Yanfei wrote:
> 
> 
> On 4/15/21 11:43 PM, Paul E. McKenney wrote:
> > [Please note: This e-mail is from an EXTERNAL e-mail address]
> > 
> > On Thu, Apr 15, 2021 at 11:04:05PM +0800, Xu, Yanfei wrote:
> > > Hi experts,
> > > 
> > > I am learning rcu mechanism and its codes. When looking at the
> > > rcu_blocking_is_gp(), I found there is a pair preemption disable/enable
> > > operation in non-preemption code path. And it has been a long time. I can't
> > > understand why we need it? Is there some thing I missed? If not, can we
> > > remove the unnecessary operation like blow?
> > 
> > Good point, you are right that preemption is disabled anyway in that block
> > of code.  However, preempt_disable() and preempt_enable() also prevent the
> > compiler from moving that READ_ONCE() around.  So my question to you is
> > whether it is safe to remove those statements entirely or whether they
> > should instead be replaced by barrier() or similar.
> 
> Thanks for your reply! :)
> 
> Yes, preempt_disable() and preempt_enable() defined in !preemption are
> barrier(). barrier can prevent from reordering that READ_ONCE(), but base on
> my current understanding, volatile in READ_ONCE can also tell the compiler
> not to reorder it. So, I think it's safe?

Maybe.

Please keep in mind that although the compiler is prohibited from
reordering volatile accesses with each other, there is nothing stopping
it from reordering volatile accesses with non-volatile accesses.

							Thanx, Paul

> Best regards,
> Yanfei
> 
> > 
> >                                                          Thanx, Paul
> > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index da6f5213fb74..c6d95a00715e 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3703,7 +3703,6 @@ static int rcu_blocking_is_gp(void)
> > >          if (IS_ENABLED(CONFIG_PREEMPTION))
> > >                  return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
> > >          might_sleep();  /* Check for RCU read-side critical section. */
> > > -       preempt_disable();
> > >          /*
> > >           * If the rcu_state.n_online_cpus counter is equal to one,
> > >           * there is only one CPU, and that CPU sees all prior accesses
> > > @@ -3718,7 +3717,6 @@ static int rcu_blocking_is_gp(void)
> > >           * Those memory barriers are provided by CPU-hotplug code.
> > >           */
> > >          ret = READ_ONCE(rcu_state.n_online_cpus) <= 1;
> > > -       preempt_enable();
> > >          return ret;
> > >   }
> > > 
> > > 
> > > 
> > > Best regards,
> > > Yanfei