Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for rwsem_down_write_failed()

[Peter Zijlstra]

On Sun, May 08, 2016 at 10:36:21PM -0700, Peter Hurley wrote:
> On 05/08/2016 09:56 PM, Davidlohr Bueso wrote:
> > The field is obviously updated w.o the lock and needs a READ_ONCE
> > while waiting for lock holder(s) to go away, just like we do with
> > all other ->count accesses.
> 
> This isn't actually fixing a bug because it's passed through
> several full barriers which will force reloading from sem->count.
> 
> I think the patch is ok if you want it just for consistency anyway,
> but please change $subject and changelog.

Agreed, and note that the READ_ONCE does prohibit load-tearing, while
the current code does not.

So in that respect the patched code is more strict. But yes, the current
code does not allow using a stale value of sem->count() because, as
PeterH notes, we've just passed through at least the full memory barrier
implied by set_current_state().