Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup

[Waiman Long]

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