linux-kernel - Re: [PATCH v6 8/8] nouveau/svm: Implement atomic SVM access

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Date:   Mon, 22 Mar 2021 20:27:33 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     <linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
        <bskeggs@...hat.com>, <akpm@...ux-foundation.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kvm-ppc@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <jhubbard@...dia.com>, <rcampbell@...dia.com>,
        <jglisse@...hat.com>, <jgg@...dia.com>, <daniel@...ll.ch>,
        <willy@...radead.org>
Subject: Re: [PATCH v6 8/8] nouveau/svm: Implement atomic SVM access

On Monday, 15 March 2021 6:51:13 PM AEDT Christoph Hellwig wrote:
> > -	/*XXX: atomic? */
> > -	return (fa->access == 0 || fa->access == 3) -
> > -	       (fb->access == 0 || fb->access == 3);
> > +	/* Atomic access (2) has highest priority */
> > +	return (-1*(fa->access == 2) + (fa->access == 0 || fa->access == 3)) -
> > +	       (-1*(fb->access == 2) + (fb->access == 0 || fb->access == 3));
> 
> This looks really unreabable.  If the magic values 0, 2 and 3 had names
> it might become a little more understadable, then factor the duplicated
> calculation of the priority value into a helper and we'll have code that
> mere humans can understand..

Fair enough, will add some definitions for the magic values.

> > +		mutex_lock(&svmm->mutex);
> > +		if (mmu_interval_read_retry(&notifier->notifier,
> > +					    notifier_seq)) {
> > +			mutex_unlock(&svmm->mutex);
> > +			continue;
> > +		}
> > +		break;
> > +	}
> 
> This looks good, why not:
> 
> 		mutex_lock(&svmm->mutex);
> 		if (!mmu_interval_read_retry(&notifier->notifier,
> 					     notifier_seq))
> 			break;
> 		mutex_unlock(&svmm->mutex);
> 	}

I had copied that from nouveau_range_fault() but this suggestion is better. 
Will update, thanks for looking.