[PATCH v3 7/9] nfs: Use d_alloc_trylock() in nfs_prime_dcache()

[NeilBrown <neilb@ownmail.net>]

From: NeilBrown <neil@brown.name>

NFS uses the results of readdir to prime the dcache.  Using
d_alloc_parallel() can block if there is a concurrent lookup.  Blocking
in that case is pointless as the lookup will add info to the dcache and
there is no value in the readdir waiting to see if it should add the
info too.

Also this call to d_alloc_parallel() is made while the parent
directory is locked.  A proposed change to locking will lock the parent
later, after d_alloc_parallel().  This means it won't be safe to wait in
d_alloc_parallel() while holding the directory lock.

So change to use d_alloc_trylock(), which removes the need for
calculating the hash or doing a preliminary lookup.

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/nfs/dir.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 8a98247b6b2d..49283f533d6e 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -749,15 +749,12 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry,
 		if (filename.len == 2 && filename.name[1] == '.')
 			return;
 	}
-	filename.hash = full_name_hash(parent, filename.name, filename.len);
 
-	dentry = d_lookup(parent, &filename);
 again:
-	if (!dentry) {
-		dentry = d_alloc_parallel(parent, &filename);
-		if (IS_ERR(dentry))
-			return;
-	}
+	dentry = d_alloc_trylock(parent, &filename);
+	if (IS_ERR(dentry))
+		return;
+
 	if (!d_in_lookup(dentry)) {
 		/* Is there a mountpoint here? If so, just exit */
 		if (!nfs_fsid_equal(&NFS_SB(dentry->d_sb)->fsid,
-- 
2.50.0.107.gf914562f5916.dirty