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

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Message-ID: <20180927111354.GA16469@quack2.suse.cz>
Date:   Thu, 27 Sep 2018 13:13:54 +0200
From:   Jan Kara <jack@...e.cz>
To:     Barret Rhoden <brho@...gle.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, 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()

On Mon 24-09-18 11:57:21, Barret Rhoden wrote:
> 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.

Yep, that looks certainly wrong. I'll send a fix.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR