Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS

[Waiman Long]

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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