Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning is enabled

[Jason Low]

On Thu, 2016-08-11 at 11:40 -0400, Waiman Long wrote:
> On 08/10/2016 02:44 PM, Jason Low wrote:
> > +static inline void do_yield_to_waiter(struct mutex *lock, int *wakeups)
> > +{
> > +	return;
> > +}
> > +
> > +static inline void clear_yield_to_waiter(struct mutex *lock)
> > +{
> > +	return;
> > +}
> > +
> > +static inline bool need_yield_to_waiter(struct mutex *lock)
> > +{
> > +	return false;
> > +}
> > +
> >   #else
> >   static bool mutex_optimistic_spin(struct mutex *lock,
> >   				  struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
> >   {
> >   	return false;
> >   }
> > +
> > +#define MUTEX_WAKEUP_THRESHOLD 16
> > +
> > +static inline void do_yield_to_waiter(struct mutex *lock, int *wakeups)
> > +{
> > +	*wakeups += 1;
> > +
> > +	if (*wakeups<  MUTEX_WAKEUP_THRESHOLD)
> > +		return;
> > +
> > +	if (lock->yield_to_waiter != true)
> > +		lock->yield_to_waiter = true;
> > +}
> > +
> > +static inline void clear_yield_to_waiter(struct mutex *lock)
> > +{
> > +	lock->yield_to_waiter = false;
> > +}
> > +
> > +static inline bool need_yield_to_waiter(struct mutex *lock)
> > +{
> > +	return lock->yield_to_waiter;
> > +}
> >   #endif
> >
> >   _
> 
> The *yield* helper functions should be in a separate conditional 
> compilation block as the declaration of yield_to_waiter may not match 
> the helper functions with certain combination of config variables.
> 
> Something like
> 
> #if !defined(CONFIG_MUTEX_SPIN_ON_OWNER) && defined(CONFIG_SMP)
> ...
> #else
> ...
> #endif

Right, we will need to incorporate the CONFIG_SMP logic when defining
these functions here, otherwise they would be undefined in the !SMP
case. 

Thanks,
Jason