Hi Dan -

On 2018-07-04 at 14:41 Dan Williams <dan.j.willi...@intel.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;
> +}

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