linux-kernel - Re: [PATCH tip/locking/core v8 1/5] locking/qspinlock: Use _acquire/

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Date:	Tue, 20 Oct 2015 11:48:49 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Waiman Long <Waiman.Long@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH tip/locking/core v8 1/5] locking/qspinlock: Use
 _acquire/_release versions of cmpxchg & xchg

On Mon, Oct 19, 2015 at 08:46:02PM -0700, Davidlohr Bueso wrote:
> On Tue, 20 Oct 2015, Boqun Feng wrote:
> 
> >>@@ -93,7 +94,7 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock)
> >> 	/*
> >> 	 * smp_mb__before_atomic() in order to guarantee release semantics
> >> 	 */
> >>-	smp_mb__before_atomic_dec();
> >>+	smp_mb__before_atomic();
> >> 	atomic_sub(_Q_LOCKED_VAL, &lock->val);
> >
> >Just be curious, you don't use atomic_sub_release() here on purpose?
> 
> atomic_sub() does not imply barriers, so there's no relaxed variants; that's
> only for _return() (and such) to the caller.
> 

Ah.. my mistake ;-(

Thank you.

Regards,
Boqun

> Thanks,
> Davidlohr



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