Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

From: Oleg Nesterov
Date: Fri Sep 18 2020 - 09:09:25 EST

On 09/18, Peter Zijlstra wrote:
> On Fri, Sep 18, 2020 at 12:48:24PM +0200, Oleg Nesterov wrote:
> > Of course, this assumes that atomic_t->counter underflows "correctly", just
> > like "unsigned int".
> We're documented that we do. Lots of code relies on that.
> See Documentation/atomic_t.txt TYPES

Aha, thanks!

> > But again, do we really want this?
> I like the two counters better, avoids atomics entirely, some archs
> hare horridly expensive atomics (*cough* power *cough*).

I meant... do we really want to introduce percpu_up_read_irqsafe() ?

Perhaps we can live with the fix from Hou? At least until we find a
"real" performance regression.