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

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Message-ID: <20160602111505.GB3190@twins.programming.kicks-ass.net>
Date:	Thu, 2 Jun 2016 13:15:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
Cc:	Arnd Bergmann <arnd@...db.de>, 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 07:01:17PM +0800, xinhui wrote:
> 
> On 2016年06月02日 18:44, 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, a little more expensive than the existing version

Think 20+ cycles worse.

> But does this is generic code, I am not sure how it will impact the performance on other archs.

As always, you get to audit users of stuff you change. And here you're
lucky, there's only 1.

> If you like
> we calculate the correct address to set to NULL
> say,
> static inline void queued_write_unlock(struct qrwlock *lock)
> {
> u8 *wl = lock;
> 
> #ifdef __BIG_ENDIAN
> wl += 3;
> #endif
> smp_store_release(wl, 0);
> 
> }

No, that's horrible. Either lift __qrwlock into qrwlock_types.h or do
what qspinlock does. And looking at that, we could make
queued_spin_unlock() use the atomic_sub_return_relaxed() thing too I
suppose, that generates slightly better code.