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

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Date:	Mon, 19 Oct 2015 20:46:02 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Boqun Feng <boqun.feng@...il.com>
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 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.

Thanks,
Davidlohr
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