xfs: prevent lockdep false positive in xfs_iget_cache_miss

The inode can't be locked by anyone else as we just created it a few
lines above and it's not been added to any lookup data structure yet.

So use a trylock that must succeed to get around the lockdep warnings.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>

fs/xfs/xfs_iget.c

@@ -246,17 +246,23 @@ xfs_iget_cache_miss(
 		goto out_destroy;
 	}
 
-	if (lock_flags)
-		xfs_ilock(ip, lock_flags);
-
 	/*
 	 * Preload the radix tree so we can insert safely under the
 	 * write spinlock. Note that we cannot sleep inside the preload
 	 * region.
 	 */
 	if (radix_tree_preload(GFP_KERNEL)) {
 		error = EAGAIN;
-		goto out_unlock;
+		goto out_destroy;
+	}
+
+	/*
+	 * Because the inode hasn't been added to the radix-tree yet it can't
+	 * be found by another thread, so we can do the non-sleeping lock here.
+	 */
+	if (lock_flags) {
+		if (!xfs_ilock_nowait(ip, lock_flags))
+			BUG();
 	}
 
 	mask = ~(((XFS_INODE_CLUSTER_SIZE(mp) >> mp->m_sb.sb_inodelog)) - 1);
@@ -284,7 +290,6 @@ xfs_iget_cache_miss(
 out_preload_end:
 	write_unlock(&pag->pag_ici_lock);
 	radix_tree_preload_end();
-out_unlock:
 	if (lock_flags)
 		xfs_iunlock(ip, lock_flags);
 out_destroy: