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

Peter Zijlstra
2017-06-06T10:53:43Z

On Mon, Jun 05, 2017 at 03:09:50PM -0700, Paul E. McKenney wrote:
> There would be a slowdown if 1) fast this_cpu_inc is not available and
> cannot be implemented (this usually means that atomic_inc has implicit
> memory barriers),

I don't get this.

How is per-cpu crud related to being strongly ordered?

this_cpu_ has 3 forms:

x86: single instruction
arm64,s390: preempt_disable()+atomic_op
generic: local_irq_save()+normal_op

Only s390 is TSO, arm64 is very much a weak arch.

> and 2) local_irq_save/restore is slower than disabling
> preemption. The main architecture with these constraints is s390, which
> however is already paying the price in __srcu_read_unlock and has not
> complained.

IIRC only PPC (and hopefully soon x86) has a local_irq_save() that is as
fast as preempt_disable().

> A valid optimization on s390 would be to skip the smp_mb;
> AIUI, this_cpu_inc implies a memory barrier (!) due to its implementation.

You mean the s390 this_cpu_inc() in specific, right? Because
this_cpu_inc() in general does not imply any such thing.