Re: [PATCH v11 09/16] qspinlock, x86: Allow unfair spinlock in a virtual guest

From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	linux-arch@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
	Paolo Bonzini <paolo.bonzini@gmail.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Rik van Riel <riel@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	David Vrabel <david.vrabel@citrix.com>,
	Oleg Nesterov <oleg@redhat.com>, Gleb Natapov <gleb@redhat.com>,
	Scott J Norton <scott.norton@hp.com>,
	Chegu Vinod <chegu_vinod@hp.com>
Subject: Re: [PATCH v11 09/16] qspinlock, x86: Allow unfair spinlock in a virtual guest
Date: Wed, 11 Jun 2014 13:38:41 +0200	[thread overview]
Message-ID: <20140611113841.GD3588@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20140611105402.GL3213@twins.programming.kicks-ass.net>

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On Wed, Jun 11, 2014 at 12:54:02PM +0200, Peter Zijlstra wrote:
> > @@ -252,6 +260,18 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> >  
> >  	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
> >  
> > +#ifdef CONFIG_VIRT_UNFAIR_LOCKS
> > +	/*
> > +	 * A simple test and set unfair lock
> > +	 */
> > +	if (static_key_false(&virt_unfairlocks_enabled)) {
> > +		cpu_relax();	/* Relax after a failed lock attempt */
> 
> Meh, I don't think anybody can tell the difference if you put that in or
> not, therefore don't.
> 
> > +		while (!queue_spin_trylock(lock))
> > +			cpu_relax();
> > +		return;
> > +	}
> > +#endif /* CONFIG_VIRT_UNFAIR_LOCKS */
> 
> If you're really worried about those upper 24bits, you can always clear
> them when you get here.

I don't think its a problem at all; flipping the static_key requires
stop_machine, which guarantees us that there are no spinlocks held. So I
think you can actually BUG_ON() the upper 24bits.

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