Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

[Heiko Carstens <heiko.carstens@de.ibm.com>]

On Tue, Jun 06, 2017 at 06:15:51PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 06, 2017 at 05:27:06PM +0200, Heiko Carstens wrote:
> > On Tue, Jun 06, 2017 at 04:45:57PM +0200, Christian Borntraeger wrote:
> 
> > > As a side note, I am asking myself, though, why we do need the
> > > preempt_disable/enable for the cases where we use the opcodes 
> > > like lao (atomic load and or to a memory location) and friends.
> > 
> > Because you want the atomic instruction to be executed on the local cpu for
> > which you have to per cpu pointer. If you get preempted to a different cpu
> > between the ptr__ assignment and lan instruction it might be executed not
> > on the local cpu. It's not really a correctness issue.
> 
> As per the previous email, I think it is a correctness issue wrt CPU
> hotplug.

Yes, I realized that just a minute after I sent the above.

> > However in reality it doesn't matter at all, since all distributions we
> > care about have preemption disabled.
> 
> Well, either you support PREEMPT=y or you don't :-) If you do, it needs
> to be correct, irrespective of what distro's do with it.

That is true. Our s390 specific percpu ops are supposed to be correct for
PREEMPT=y, and that's apparently the only reason why I added the preempt
disable/enable pairs back then. I just had to remember why I did that ;)

> > So this_cpu_inc() should just generate three instructions: two to calculate
> > the percpu pointer and an additional asi for the atomic increment, with
> > operand specific serialization. This is supposed to be a lot faster than
> > disabling/enabling interrupts around a non-atomic operation.
> 
> So typically we joke about s390 that it has an instruction for this
> 'very-complicated-thing', but here you guys do not, what gives? ;-)

Tough luck. :)