linux-kernel - Re: [PATCH v10 07/19] qspinlock: Use a simple write to grab the lock, if applicable

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Date:	Fri, 09 May 2014 21:05:25 -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 07/19] qspinlock: Use a simple write to grab the lock,
 if applicable

On 05/08/2014 03:00 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 11:01:35AM -0400, Waiman Long wrote:
>> @@ -94,23 +94,29 @@ static inline struct mcs_spinlock *decode_tail(u32 tail)
>>    * can allow better optimization of the lock acquisition for the pending
>>    * bit holder.
>>    */
>> -#if _Q_PENDING_BITS == 8
>> -
>>   struct __qspinlock {
>>   	union {
>>   		atomic_t val;
>> -		struct {
>>   #ifdef __LITTLE_ENDIAN
>> +		u8	 locked;
>> +		struct {
>>   			u16	locked_pending;
>>   			u16	tail;
>> +		};
>>   #else
>> +		struct {
>>   			u16	tail;
>>   			u16	locked_pending;
>> -#endif
>>   		};
>> +		struct {
>> +			u8	reserved[3];
>> +			u8	locked;
>> +		};
>> +#endif
>>   	};
>>   };
>>
>> +#if _Q_PENDING_BITS == 8
> That doesn't make sense, that struct __qspinlock only makes sense when
> _Q_PENDING_BITS == 8.

I need to use the locked field (the 2nd strcut) in get_qlock() where I 
grab the lock by setting the lock byte directly. Since the endian-aware 
structure is already in place, I reused it and have to expose it even 
when _Q_PENDING_BITS isn't 8. I will document that more clearly in the 
code to avoid this confusion.

-Longman
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