linux-kernel - Re: [PATCH v6 1/6] locking/qspinlock: relaxes cmpxchg & xchg ops in native code

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Message-ID: <20150914120619.GQ18489@twins.programming.kicks-ass.net>
Date:	Mon, 14 Sep 2015 14:06:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Waiman Long <Waiman.Long@....com>, 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 v6 1/6] locking/qspinlock: relaxes cmpxchg & xchg ops in
 native code

On Fri, Sep 11, 2015 at 03:27:44PM -0700, Davidlohr Bueso wrote:
> On Fri, 11 Sep 2015, Waiman Long wrote:
> 
> >@@ -46,7 +46,7 @@ static inline bool virt_queued_spin_lock(struct qspinlock *lock)
> >	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
> >		return false;
> >
> >-	while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0)
> >+	while (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) != 0)
> >		cpu_relax();
> 
> This code has changed with Peter's recent ccas fix. And the whole virt_queued_spin_lock()
> thing will now be under pv configs. So this doesn't apply to native code anymore, so it
> looks like it should be dropped altogether.

Yeah, it also doesn't make sense, this ix x86 arch code, x86 cannot do
cmpxchg_acquire. Then again, I suppose we could argue its of
documentation value..
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