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

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Message-ID: <20160602110200.GZ3190@twins.programming.kicks-ass.net>
Date:	Thu, 2 Jun 2016 13:02:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	waiman.long@...com
Subject: Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian

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).