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SubjectRe: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup
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On 4/26/21 4:08 AM, Peter Zijlstra wrote:
> On Sun, Apr 25, 2021 at 04:06:37PM -0400, Waiman Long wrote:
>
>> void queued_write_lock_slowpath(struct qrwlock *lock)
>> {
>> - int cnts;
>> + int cnts = 0;
>>
>> /* Put the writer into the wait queue */
>> arch_spin_lock(&lock->wait_lock);
>>
>> /* Try to acquire the lock directly if no reader is present */
>> if (!atomic_read(&lock->cnts) &&
>> - (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
>> + atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
>> goto unlock;
> Would not something like:
>
> if (!(cnts = atomic_read(&lock->cnts)) &&
> atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)
> goto unlock;
>
> Be clearer?
That works for me too. It is equivalent anyway.
>
>>
>> - /* Set the waiting flag to notify readers that a writer is pending */
>> - atomic_add(_QW_WAITING, &lock->cnts);
>> + /*
>> + * Set the waiting flag to notify readers that a writer is pending
>> + *
>> + * As only one writer who is the wait_lock owner can set the waiting
>> + * flag which will be cleared later on when acquiring the write lock,
>> + * we can easily replace atomic_or() by an atomic_add() if there is
>> + * an architecture where an atomic_add() performs better than an
>> + * atomic_or().
> That might be a little overboard on the comment, but sure :-) I don't
> think there's any arch that doesn't have atomic_or(), like I wrote
> elsewhere, the one that's often an issue is atomic_fetch_or().
>
I was not sure as I didn't look at other archs that hadn't used qrwlock
yet. Given what you said, I will remove the comment.

Cheers,
Longman

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