linux-kernel - Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR

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Date:	Sat, 19 Apr 2014 11:24:42 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS


* Waiman Long <waiman.long@...com> wrote:

> On 04/18/2014 03:46 AM, Ingo Molnar wrote:
> >* Waiman Long<waiman.long@...com>  wrote:
> >
> >>On 04/17/2014 11:58 AM, Peter Zijlstra wrote:
> >>>On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
> >>>>+static __always_inline void
> >>>>+clear_pending_set_locked(struct qspinlock *lock, u32 val)
> >>>>+{
> >>>>+	struct __qspinlock *l = (void *)lock;
> >>>>+
> >>>>+	ACCESS_ONCE(l->locked_pending) = 1;
> >>>>+}
> >>>>@@ -157,8 +251,13 @@ static inline int trylock_pending(struct qspinlock *lock, u32 *pval)
> >>>>  	 * we're pending, wait for the owner to go away.
> >>>>  	 *
> >>>>  	 * *,1,1 ->   *,1,0
> >>>>+	 *
> >>>>+	 * this wait loop must be a load-acquire such that we match the
> >>>>+	 * store-release that clears the locked bit and create lock
> >>>>+	 * sequentiality; this because not all try_clear_pending_set_locked()
> >>>>+	 * implementations imply full barriers.
> >>>You renamed the function referred in the above comment.
> >>>
> >>Sorry, will fix the comments.
> >I suggest not renaming the function instead.
> >try_clear_pending_set_locked() tells the intent in a clearer fashion.
> >
> >Thanks,
> >
> >	Ingo
> 
> I usually use the word "try" if there is a possibility of failure. 
> However, the function will always succeed, albeit by waiting a bit 
> in some cases. That is why I remove "try" from the name.

Fair enough!

Thanks,

	Ingo
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