linux-kernel - Re: [PATCH v10 10/19] qspinlock, x86: Allow unfair spinlock in a virtual guest

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Date:	Mon, 19 May 2014 16:30:22 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	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>,
	Boris Ostrovsky <boris.ostrovsky@...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 v10 10/19] qspinlock, x86: Allow unfair spinlock in a
 virtual guest

On 05/08/2014 03:12 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 11:01:38AM -0400, Waiman Long wrote:
>
>
> No, we want the unfair thing for VIRT, not PARAVIRT.
>

Yes, you are right. I will change that to VIRT.

>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index 9e7659e..10e87e1 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -227,6 +227,14 @@ static __always_inline int get_qlock(struct qspinlock *lock)
>>   {
>>   	struct __qspinlock *l = (void *)lock;
>>
>> +#ifdef CONFIG_PARAVIRT_UNFAIR_LOCKS
>> +	if (static_key_false(&paravirt_unfairlocks_enabled))
>> +		/*
>> +		 * Need to use atomic operation to get the lock when
>> +		 * lock stealing can happen.
>> +		 */
>> +		return cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0;
> That's missing {}.

It is a single statement which doesn't need braces according to kernel 
coding style. I could move the comments up a bit to make it easier to read.

>> +#endif
>
>>   	barrier();
>>   	ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
>>   	barrier();
>
> But no, what you want is:
>
> static __always_inline bool virt_lock(struct qspinlock *lock)
> {
> #ifdef CONFIG_VIRT_MUCK
> 	if (static_key_false(&virt_unfairlocks_enabled)) {
> 		while (!queue_spin_trylock(lock))
> 			cpu_relax();
>
> 		return true;
> 	}
> #else
> 	return false;
> }
>
>
> void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> {
> 	if (virt_lock(lock))
> 		return;
>
> 	...
> }

This is a possible way of doing it. I can do that in the patch series to 
simplify it. Hopefully that will speed up the review process and get it 
done quicker.

-Longman
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