Re: [PATCH 07/11] qspinlock: Use a simple write to grab the lock, if applicable

From: Peter Zijlstra <peterz@infradead.org>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Waiman.Long@hp.com, linux-arch@vger.kernel.org, gleb@redhat.com,
	kvm@vger.kernel.org, boris.ostrovsky@oracle.com,
	scott.norton@hp.com, raghavendra.kt@linux.vnet.ibm.com,
	paolo.bonzini@gmail.com, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, chegu_vinod@hp.com,
	david.vrabel@citrix.com, oleg@redhat.com,
	xen-devel@lists.xenproject.org, tglx@linutronix.de,
	paulmck@linux.vnet.ibm.com, torvalds@linux-foundation.org,
	mingo@kernel.org
Subject: Re: [PATCH 07/11] qspinlock: Use a simple write to grab the lock, if applicable
Date: Mon, 7 Jul 2014 16:51:45 +0200	[thread overview]
Message-ID: <20140707145145.GT6758__40527.9908598915$1404744830$gmane$org@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20140618163615.GA5331@laptop.dumpdata.com>

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On Wed, Jun 18, 2014 at 12:36:15PM -0400, Konrad Rzeszutek Wilk wrote:
> On Sun, Jun 15, 2014 at 02:47:04PM +0200, Peter Zijlstra wrote:
> > From: Waiman Long <Waiman.Long@hp.com>
> > 
> > Currently, atomic_cmpxchg() is used to get the lock. However, this is
> > not really necessary if there is more than one task in the queue and
> > the queue head don't need to reset the queue code word. For that case,
> 
> s/queue code word/tail {number,value}/ ?
> 
> 
> > a simple write to set the lock bit is enough as the queue head will
> > be the only one eligible to get the lock as long as it checks that
> > both the lock and pending bits are not set. The current pending bit
> > waiting code will ensure that the bit will not be set as soon as the
> > queue code word (tail) in the lock is set.
> 
> Just use the same word as above.

I made that:

Currently, atomic_cmpxchg() is used to get the lock. However, this is
not really necessary if there is more than one task in the queue and
the queue head doesn't need to reset the queue tail.

For that case, a simple write to set the lock byte is enough as the
queue head will be the only one eligible to get the lock as long as it
checks that both the lock and pending bits are not set. The current
pending bit waiting code will ensure that the bit will not be set as
soon as the queue tail is set.

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