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>

fs/dax.c

@@ -373,6 +373,22 @@ restart:
 		}
 		spin_lock_irq(&mapping->tree_lock);
 
+		if (!entry) {
+			/*
+			 * We needed to drop the page_tree lock while calling
+			 * radix_tree_preload() and we didn't have an entry to
+			 * lock.  See if another thread inserted an entry at
+			 * our index during this time.
+			 */
+			entry = __radix_tree_lookup(&mapping->page_tree, index,
+					NULL, &slot);
+			if (entry) {
+				radix_tree_preload_end();
+				spin_unlock_irq(&mapping->tree_lock);
+				goto restart;
+			}
+		}
+
 		if (pmd_downgrade) {
 			radix_tree_delete(&mapping->page_tree, index);
 			mapping->nrexceptional--;
@@ -388,19 +404,12 @@ restart:
 		if (err) {
 			spin_unlock_irq(&mapping->tree_lock);
 			/*
-			 * Someone already created the entry?  This is a
+			 * Our insertion of a DAX entry failed, most likely
-			 * normal failure when inserting PMDs in a range
+			 * because we were inserting a PMD entry and it
-			 * that already contains PTEs.  In that case we want
+			 * collided with a PTE sized entry at a different
-			 * to return -EEXIST immediately.
+			 * index in the PMD range.  We haven't inserted
-			 */
+			 * anything into the radix tree and have no waiters to
-			if (err == -EEXIST && !(size_flag & RADIX_DAX_PMD))
+			 * wake.
-				goto restart;
-			/*
-			 * Our insertion of a DAX PMD entry failed, most
-			 * likely because it collided with a PTE sized entry
-			 * at a different index in the PMD range.  We haven't
-			 * inserted anything into the radix tree and have no
-			 * waiters to wake.
 			 */
 			return ERR_PTR(err);
 		}