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

[Waiman Long]

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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