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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH] locking/qspinlock: Fix typo of lock word transition in
 the uncontended case
To:     Will Deacon <will@kernel.org>, Zenghui Yu <yuzenghui@huawei.com>
Cc:     peterz@infradead.org, mingo@redhat.com, boqun.feng@gmail.com,
        wanghaibin.wang@huawei.com, linux-kernel@vger.kernel.org
References: <20210715030847.2038-1-yuzenghui@huawei.com>
 <20210809134047.GB1207@willie-the-truck>
Message-ID: <7e71b365-e204-d9d6-39ef-ef4f08f2af18@redhat.com>
Date:   Mon, 9 Aug 2021 12:22:31 -0400
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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@huawei.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@kernel.org>
>
> 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@redhat.com>