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

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Message-ID: <20210809134047.GB1207@willie-the-truck>
Date:   Mon, 9 Aug 2021 14:40:48 +0100
From:   Will Deacon <will@...nel.org>
To:     Zenghui Yu <yuzenghui@...wei.com>
Cc:     peterz@...radead.org, mingo@...hat.com, longman@...hat.com,
        boqun.feng@...il.com, wanghaibin.wang@...wei.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/qspinlock: Fix typo of lock word transition in
 the uncontended case

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@...wei.com>
> ---
>  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@...nel.org>

Then again, I acked the patch introducing this comment so what do I know?

Will