dax: fix radix tree insertion race

dax: fix radix tree insertion race

While running generic/340 in my test setup I hit the following race.  It
can happen with kernels that support FS DAX PMDs, so v4.10 thru
v4.11-rc5.

Thread 1				Thread 2
--------				--------
dax_iomap_pmd_fault()
  grab_mapping_entry()
    spin_lock_irq()
    get_unlocked_mapping_entry()
    'entry' is NULL, can't call lock_slot()
    spin_unlock_irq()
    radix_tree_preload()
					dax_iomap_pmd_fault()
					  grab_mapping_entry()
					    spin_lock_irq()
					    get_unlocked_mapping_entry()
					    ...
					    lock_slot()
					    spin_unlock_irq()
					  dax_pmd_insert_mapping()
					    <inserts a PMD mapping>
    spin_lock_irq()
    __radix_tree_insert() fails with -EEXIST
    <fall back to 4k fault, and die horribly
     when inserting a 4k entry where a PMD exists>

The issue is that we have to drop mapping->tree_lock while calling
radix_tree_preload(), but since we didn't have a radix tree entry to
lock (unlike in the pmd_downgrade case) we have no protection against
Thread 2 coming along and inserting a PMD at the same index.  For 4k
entries we handled this with a special-case response to -EEXIST coming
from the __radix_tree_insert(), but this doesn't save us for PMDs
because the -EEXIST case can also mean that we collided with a 4k entry
in the radix tree at a different index, but one that is covered by our
PMD range.

So, correctly handle both the 4k and 2M collision cases by explicitly
re-checking the radix tree for an entry at our index once we reacquire
mapping->tree_lock.

This patch has made it through a clean xfstests run with the current
v4.11-rc5 based linux/master, and it also ran generic/340 500 times in a
loop.  It used to fail within the first 10 iterations.

Link: http://lkml.kernel.org/r/20170406212944.2866-1-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: <stable@vger.kernel.org>    [4.10+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>