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

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Date:	Thu, 17 Apr 2014 17:46:27 -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>,
	"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

On 04/17/2014 11:56 AM, Peter Zijlstra wrote:
> On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
>> +struct __qspinlock {
>> +	union {
>> +		atomic_t val;
>> +		struct {
>> +#ifdef __LITTLE_ENDIAN
>> +			u16	locked_pending;
>> +			u16	tail;
>> +#else
>> +			u16	tail;
>> +			u16	locked_pending;
>> +#endif
>> +		};
>> +	};
>> +};
>> +
>> +/**
>> + * clear_pending_set_locked - take ownership and clear the pending bit.
>> + * @lock: Pointer to queue spinlock structure
>> + * @val : Current value of the queue spinlock 32-bit word
>> + *
>> + * *,1,0 ->  *,0,1
>> + */
>> +static __always_inline void
>> +clear_pending_set_locked(struct qspinlock *lock, u32 val)
>> +{
>> +	struct __qspinlock *l = (void *)lock;
>> +
>> +	ACCESS_ONCE(l->locked_pending) = 1;
> You lost the __constant_le16_to_cpu(_Q_LOCKED_VAL) there. The
> unconditional 1 is wrong. You also have to flip the bytes in
> locked_pending.

I don't think that is wrong. The lock byte is in the least significant 8 
bits and the pending byte is the next higher significant 8 bits 
irrespective of the endian-ness. So a value of 1 in a 16-bit context 
means the lock byte is set, but the pending byte is cleared. The name 
"locked_pending" doesn't mean that locked variable is in a lower address 
than pending.

-Longman
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