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

[peterz]

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

> 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 just tried to be clever and use a single u64 load (where possible)
instead of two 32bit loads and got the sum vs split order wrong.