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

[Peter Zijlstra]

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.

> 
> #define arch_this_cpu_to_op(pcp, val, op)				\
> {									\
> 	typedef typeof(pcp) pcp_op_T__;					\
> 	pcp_op_T__ val__ = (val);					\
> 	pcp_op_T__ old__, *ptr__;					\
> 	preempt_disable();						\
> 	ptr__ = raw_cpu_ptr(&(pcp));					\
> 	asm volatile(							\
> 		op "	%[old__],%[val__],%[ptr__]\n"			\
> 		: [old__] "=d" (old__), [ptr__] "+Q" (*ptr__)		\
> 		: [val__] "d" (val__)					\
> 		: "cc");						\
> 	preempt_enable();						\
> }
> 
> #define this_cpu_and_4(pcp, val)	arch_this_cpu_to_op(pcp, val, "lan")
> 
> 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.

> 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? ;-)