Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian

[Peter Zijlstra]

On Thu, Jun 02, 2016 at 12:44:51PM +0200, Arnd Bergmann wrote:
> On Thursday, June 2, 2016 6:09:08 PM CEST Pan Xinhui wrote:
> > diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> > index 54a8e65..eadd7a3 100644
> > --- a/include/asm-generic/qrwlock.h
> > +++ b/include/asm-generic/qrwlock.h
> > @@ -139,7 +139,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
> >   */
> >  static inline void queued_write_unlock(struct qrwlock *lock)
> >  {
> > -       smp_store_release((u8 *)&lock->cnts, 0);
> > +       (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
> >  }
> 
> Isn't this more expensive than the existing version?

Yes, loads. And while this might be a suitable fix for asm-generic, it
will introduce a fairly large regression on x86 (which is currently the
only user of this).