Linux Virtualization - May 2016 - [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin_unlock

On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui
wrote:
> cmpxchg_release is light-wight than cmpxchg, we can gain a better
> performace then. On some arch like ppc, barrier impact the performace
> too much.
> 
> Suggested-by:  Boqun Feng <boqun.feng at gmail.com>
> Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
> ---
>  kernel/locking/qspinlock_paravirt.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/qspinlock_paravirt.h
b/kernel/locking/qspinlock_paravirt.h
> index a5b1248..2bbffe4 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock
*lock)
>  	 * unhash. Otherwise it would be possible to have multiple @lock
>  	 * entries, which would be BAD.
>  	 */
> -	locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
> +	locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0);
>  	if (likely(locked == _Q_LOCKED_VAL))
>  		return;
This patch fails to explain _why_ it can be relaxed.

And seeing how this cmpxchg() can actually unlock the lock, I don't see
how this can possibly be correct. Maybe cmpxchg_release(), but relaxed
seems very wrong.

On Thu, May 26, 2016 at 06:47:29PM +0200, Peter Zijlstra
wrote:
> On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote:
> > cmpxchg_release is light-wight than cmpxchg, we can gain a better
> > performace then. On some arch like ppc, barrier impact the performace
> > too much.
> > 
> > Suggested-by:  Boqun Feng <boqun.feng at gmail.com>
> > Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
> > ---
> >  kernel/locking/qspinlock_paravirt.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/locking/qspinlock_paravirt.h
b/kernel/locking/qspinlock_paravirt.h
> > index a5b1248..2bbffe4 100644
> > --- a/kernel/locking/qspinlock_paravirt.h
> > +++ b/kernel/locking/qspinlock_paravirt.h
> > @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct
qspinlock *lock)
> >  	 * unhash. Otherwise it would be possible to have multiple @lock
> >  	 * entries, which would be BAD.
> >  	 */
> > -	locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
> > +	locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0);
> >  	if (likely(locked == _Q_LOCKED_VAL))
> >  		return;
> 
> This patch fails to explain _why_ it can be relaxed.
> 
> And seeing how this cmpxchg() can actually unlock the lock, I don't see
> how this can possibly be correct. Maybe cmpxchg_release(), but relaxed
> seems very wrong.
Clearly I need to stop working for the day, I cannea read. You're doing
release, not relaxed.

Still Changelog needs improvement.

On 2016?05?27? 00:57, Peter Zijlstra wrote:
> On Thu, May 26, 2016 at 06:47:29PM +0200, Peter Zijlstra wrote:
>> On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote:
>>> cmpxchg_release is light-wight than cmpxchg, we can gain a better
>>> performace then. On some arch like ppc, barrier impact the
performace
>>> too much.
>>>
>>> Suggested-by:  Boqun Feng <boqun.feng at gmail.com>
>>> Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
>>> ---
>>>   kernel/locking/qspinlock_paravirt.h | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/locking/qspinlock_paravirt.h
b/kernel/locking/qspinlock_paravirt.h
>>> index a5b1248..2bbffe4 100644
>>> --- a/kernel/locking/qspinlock_paravirt.h
>>> +++ b/kernel/locking/qspinlock_paravirt.h
>>> @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct
qspinlock *lock)
>>>   	 * unhash. Otherwise it would be possible to have multiple @lock
>>>   	 * entries, which would be BAD.
>>>   	 */
>>> -	locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
>>> +	locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0);
>>>   	if (likely(locked == _Q_LOCKED_VAL))
>>>   		return;
>>
>> This patch fails to explain _why_ it can be relaxed.
>>
>> And seeing how this cmpxchg() can actually unlock the lock, I don't
see
>> how this can possibly be correct. Maybe cmpxchg_release(), but relaxed
>> seems very wrong.
>
> Clearly I need to stop working for the day, I cannea read. You're doing
> release, not relaxed.
>
Never mind.  thanks for review :)
> Still Changelog needs improvement.
>
Will do that.

thanks
xinhui