linux-kernel - Re: [PATCH v5 2/9] dax: Introduce holder for dax

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Message-ID: <20210719151744.GA22718@lst.de>
Date:   Mon, 19 Jul 2021 17:17:44 +0200
From:   Christoph Hellwig <hch@....de>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, dm-devel@...hat.com,
        darrick.wong@...cle.com, dan.j.williams@...el.com,
        david@...morbit.com, hch@....de, agk@...hat.com,
        snitzer@...hat.com, rgoldwyn@...e.de
Subject: Re: [PATCH v5 2/9] dax: Introduce holder for dax_device

On Mon, Jun 28, 2021 at 08:02:11AM +0800, Shiyang Ruan wrote:
> +int dax_holder_notify_failure(struct dax_device *dax_dev, loff_t offset,
> +			      size_t size, void *data)
> +{
> +	int rc = -ENXIO;
> +	if (!dax_dev)
> +		return rc;
> +
> +	if (dax_dev->holder_data) {
> +		rc = dax_dev->holder_ops->notify_failure(dax_dev, offset,
> +							 size, data);
> +		if (rc == -ENODEV)
> +			rc = -ENXIO;
> +	} else
> +		rc = -EOPNOTSUPP;

The style looks a little odd.  Why not:

	if (!dax_dev)
		return -ENXIO
	if (!dax_dev->holder_data)
		return -EOPNOTSUPP;
	return dax_dev->holder_ops->notify_failure(dax_dev, offset, size, data);

and let everyone deal with the same errno codes?

Also why do we even need the dax_dev NULL check?

> +void dax_set_holder(struct dax_device *dax_dev, void *holder,
> +		const struct dax_holder_operations *ops)
> +{
> +	if (!dax_dev)
> +		return;

I don't think we really need that check here.

> +void *dax_get_holder(struct dax_device *dax_dev)
> +{
> +	void *holder_data;
> +
> +	if (!dax_dev)
> +		return NULL;

Same here.

> +
> +	down_read(&dax_dev->holder_rwsem);
> +	holder_data = dax_dev->holder_data;
> +	up_read(&dax_dev->holder_rwsem);
> +
> +	return holder_data;

That lock won't protect anything.  I think we simply must have
synchronization to prevent unregistration while the ->notify_failure
call is in progress.