Virtualization - Apr 2014 - [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS

On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long
wrote:
> +static __always_inline void
> +clear_pending_set_locked(struct qspinlock *lock, u32 val)
> +{
> +	struct __qspinlock *l = (void *)lock;
> +
> +	ACCESS_ONCE(l->locked_pending) = 1;
> +}
> @@ -157,8 +251,13 @@ static inline int trylock_pending(struct qspinlock
*lock, u32 *pval)
>  	 * we're pending, wait for the owner to go away.
>  	 *
>  	 * *,1,1 -> *,1,0
> +	 *
> +	 * this wait loop must be a load-acquire such that we match the
> +	 * store-release that clears the locked bit and create lock
> +	 * sequentiality; this because not all try_clear_pending_set_locked()
> +	 * implementations imply full barriers.
You renamed the function referred in the above comment.
>  	 */
> -	while ((val = atomic_read(&lock->val)) & _Q_LOCKED_MASK)
> +	while ((val = smp_load_acquire(&lock->val.counter)) &
_Q_LOCKED_MASK)
>  		arch_mutex_cpu_relax();
>  
>  	/*

On 04/17/2014 11:58 AM, Peter Zijlstra wrote:
> On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
>> +static __always_inline void
>> +clear_pending_set_locked(struct qspinlock *lock, u32 val)
>> +{
>> +	struct __qspinlock *l = (void *)lock;
>> +
>> +	ACCESS_ONCE(l->locked_pending) = 1;
>> +}
>> @@ -157,8 +251,13 @@ static inline int trylock_pending(struct qspinlock
*lock, u32 *pval)
>>   	 * we're pending, wait for the owner to go away.
>>   	 *
>>   	 * *,1,1 ->  *,1,0
>> +	 *
>> +	 * this wait loop must be a load-acquire such that we match the
>> +	 * store-release that clears the locked bit and create lock
>> +	 * sequentiality; this because not all try_clear_pending_set_locked()
>> +	 * implementations imply full barriers.
> You renamed the function referred in the above comment.
>
Sorry, will fix the comments.

-Longman

* Waiman Long <waiman.long at hp.com> wrote:
> On 04/17/2014 11:58 AM, Peter Zijlstra wrote:
> >On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
> >>+static __always_inline void
> >>+clear_pending_set_locked(struct qspinlock *lock, u32 val)
> >>+{
> >>+	struct __qspinlock *l = (void *)lock;
> >>+
> >>+	ACCESS_ONCE(l->locked_pending) = 1;
> >>+}
> >>@@ -157,8 +251,13 @@ static inline int trylock_pending(struct
qspinlock *lock, u32 *pval)
> >>  	 * we're pending, wait for the owner to go away.
> >>  	 *
> >>  	 * *,1,1 ->  *,1,0
> >>+	 *
> >>+	 * this wait loop must be a load-acquire such that we match the
> >>+	 * store-release that clears the locked bit and create lock
> >>+	 * sequentiality; this because not all
try_clear_pending_set_locked()
> >>+	 * implementations imply full barriers.
> >You renamed the function referred in the above comment.
> >
> 
> Sorry, will fix the comments.
I suggest not renaming the function instead. 
try_clear_pending_set_locked() tells the intent in a clearer fashion.

Thanks,

	Ingo