Re: [PATCH] locking/qspinlock: Fix typo of lock word transition in the uncontended case

[Waiman Long]

On 8/9/21 9:40 AM, Will Deacon wrote:
> On Thu, Jul 15, 2021 at 11:08:47AM +0800, Zenghui Yu wrote:
> > If the queue head is the only one in the queue and nobody is concurrently
> > setting PENDING bit, the uncontended transition should be n,0,0 -> 0,0,1.
> >
> > Fix the typo.
> >
> > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > ---
> >   kernel/locking/qspinlock.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> > index cbff6ba53d56..591835415698 100644
> > --- a/kernel/locking/qspinlock.c
> > +++ b/kernel/locking/qspinlock.c
> > @@ -355,7 +355,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> >   	 * If we observe contention, there is a concurrent locker.
> >   	 *
> >   	 * Undo and queue; our setting of PENDING might have made the
> > -	 * n,0,0 -> 0,0,0 transition fail and it will now be waiting
> > +	 * n,0,0 -> 0,0,1 transition fail and it will now be waiting
> >   	 * on @next to become !NULL.
> >   	 */
> I think this is an important typo fix as you're right that we don't
> transition directly from having a waitqueue installed in the tail straight
> to an unlocked state.
>
> Acked-by: Will Deacon <will@xxxxxxxxxx>
>
> Then again, I acked the patch introducing this comment so what do I know?

We usually focus more on the actual code than the associated comment. I 
am not surprise we may miss that. I do agree that the proposed change is 
better.

Acked-by: Waiman Long <longman@xxxxxxxxxx>