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

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Date:   Mon, 15 Mar 2021 07:51:13 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Alistair Popple <apopple@...dia.com>
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, hch@...radead.org, daniel@...ll.ch,
        willy@...radead.org
Subject: Re: [PATCH v6 8/8] nouveau/svm: Implement atomic SVM access

> -	/*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..

> +		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);
	}