Re: [PATCH v2] rwsem: Support optimistic spinning

[Jason Low]

On Wed, Apr 30, 2014 at 2:04 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
>> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>> +{
>> +     int retval;
>
> And yet the return value is bool.
>
>> +     struct task_struct *owner;
>> +
>> +     rcu_read_lock();
>> +     owner = ACCESS_ONCE(sem->owner);
>> +
>> +     /* Spin only if active writer running */
>> +     if (owner)
>> +             retval = owner->on_cpu;
>> +     else {
>> +             /*
>> +              * When the owner is not set, the sem owner may have just
>> +              * acquired it and not set the owner yet, or the sem has
>> +              * been released, or reader active.
>> +              */
>> +             retval = false;
>> +     }
>
> And if you init the retval to false, you can leave this entire branch
> out.
>
>> +     rcu_read_unlock();
>> +
>> +     return retval;
>> +}
>
>
> Which yields the much shorter:
>
> static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> {
>         struct task_struct *owner;
>         bool on_cpu = false;

Wouldn't we want to initialize on_cpu = true. For the !owner case, I
would expect that we want to spin for the lock.

>         rcu_read_lock();
>         owner = ACCESS_ONCE(sem->owner);
>         if (owner)
>                 on_cpu = owner->on_cpu;
>         rcu_read_unlock();
>
>         return on_cpu;
> }
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/