linux-kernel - Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

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Message-ID: <20150714093148.GY18673@twins.programming.kicks-ass.net>
Date:	Tue, 14 Jul 2015 11:31:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	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 1/7] locking/pvqspinlock: Only kick CPU at unlock time

On Mon, Jul 13, 2015 at 03:48:28PM +0200, Peter Zijlstra wrote:
> @@ -239,9 +265,16 @@ static void pv_wait_head(struct qspinloc
>  			cpu_relax();
>  		}
>  
> -		WRITE_ONCE(pn->state, vcpu_halted);
> +		/*
> +		 * Either pv_kick_node() advanced us and ->state is already
> +		 * vcpu_hashed and this store is superfluous, or this is the
> +		 * first in which case the below cmpxchg() provides the
> +		 * required barriers.
> +		 */
> +		WRITE_ONCE(pn->state, vcpu_hashed);

The easier option is of course to just move this store into the branch
below. That immediately clarifies the logic and avoids the superfluous
store.

>  		if (!lp) { /* ONCE */
>  			lp = pv_hash(lock, pn);
> +
>  			/*
>  			 * lp must be set before setting _Q_SLOW_VAL
>  			 *
--
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