linux-kernel - Re: [PATCH v5 07/11] filesystem-dax: Introduce dax_lock_mapping

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Message-ID: <20180924115721.75893931@gnomeregan.cam.corp.google.com>
Date:   Mon, 24 Sep 2018 11:57:21 -0400
From:   Barret Rhoden <brho@...gle.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm@...ts.01.org, hch@....de,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jack@...e.cz,
        ross.zwisler@...ux.intel.com
Subject: Re: [PATCH v5 07/11] filesystem-dax: Introduce
 dax_lock_mapping_entry()

Hi Dan -

On 2018-07-04 at 14:41 Dan Williams <dan.j.williams@...el.com> wrote:
[snip]
> diff --git a/fs/dax.c b/fs/dax.c
> index 4de11ed463ce..57ec272038da 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
[snip]
> +bool dax_lock_mapping_entry(struct page *page)
> +{
> +	pgoff_t index;
> +	struct inode *inode;
> +	bool did_lock = false;
> +	void *entry = NULL, **slot;
> +	struct address_space *mapping;
> +
> +	rcu_read_lock();
> +	for (;;) {
> +		mapping = READ_ONCE(page->mapping);
> +
> +		if (!dax_mapping(mapping))
> +			break;
> +
> +		/*
> +		 * In the device-dax case there's no need to lock, a
> +		 * struct dev_pagemap pin is sufficient to keep the
> +		 * inode alive, and we assume we have dev_pagemap pin
> +		 * otherwise we would not have a valid pfn_to_page()
> +		 * translation.
> +		 */
> +		inode = mapping->host;
> +		if (S_ISCHR(inode->i_mode)) {
> +			did_lock = true;
> +			break;
> +		}
> +
> +		xa_lock_irq(&mapping->i_pages);
> +		if (mapping != page->mapping) {
> +			xa_unlock_irq(&mapping->i_pages);
> +			continue;
> +		}
> +		index = page->index;
> +
> +		entry = __get_unlocked_mapping_entry(mapping, index, &slot,
> +				entry_wait_revalidate);
> +		if (!entry) {
> +			xa_unlock_irq(&mapping->i_pages);
> +			break;
> +		} else if (IS_ERR(entry)) {
> +			WARN_ON_ONCE(PTR_ERR(entry) != -EAGAIN);
> +			continue;

In the IS_ERR case, do you need to xa_unlock the mapping?  It looks
like you'll deadlock the next time around the loop.

> +		}
> +		lock_slot(mapping, slot);
> +		did_lock = true;
> +		xa_unlock_irq(&mapping->i_pages);
> +		break;
> +	}
> +	rcu_read_unlock();
> +
> +	return did_lock;
> +}