Re: [Xen-devel] [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

[Waiman Long <waiman.long@xxxxxx>]

On 05/08/2014 03:06 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
> > If unfair lock is supported, the lock acquisition loop at the end of
> > the queue_spin_lock_slowpath() function may need to detect the fact
> > the lock can be stolen. Code are added for the stolen lock detection.
> >
> > A new qhead macro is also defined as a shorthand for mcs.locked.
> NAK, unfair should be a pure test-and-set lock.
I have performance data showing that a simple test-and-set lock does not 
scale well. That is the primary reason of ditching the test-and-set lock 
and use a more complicated scheme which scales better. Also, it will be 
hard to make the unfair test-and-set lock code to coexist nicely with PV 
spinlock code.
> >   /**
> >    * get_qlock - Set the lock bit and own the lock
> > - * @lock: Pointer to queue spinlock structure
> > + * @lock : Pointer to queue spinlock structure
> > + * Return: 1 if lock acquired, 0 otherwise
> >    *
> >    * This routine should only be called when the caller is the only one
> >    * entitled to acquire the lock.
> >    */
> > -static __always_inline void get_qlock(struct qspinlock *lock)
> > +static __always_inline int get_qlock(struct qspinlock *lock)
> >   {
> >         struct __qspinlock *l = (void *)lock;
> >
> >         barrier();
> >         ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
> >         barrier();
> > +       return 1;
> >   }
> and here you make a horribly named function more horrible;
> try_set_locked() is that its now.
Will do.

-Longman

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