[PATCH] namei: results of d_is_negative() should be checked after dentry revalidation

[Trond Myklebust <trond.myklebust@primarydata.com>]

Leandro Awa writes:
After switching to version 4.1.6, our parallelized and distributed workflows now  fail consistently with errors of the form:

T34: ./regex.c:39:22: error: config.h: No such file or directory

>From our 'git bisect' testing, the following commit appears to be
the possible cause of the behavior we've been seeing: commit 766c4cbfacd8

The issue is that revalidation may cause the dentry to be dropped in NFS
if, say, the client notes that the directory timestamps have changed.

Reported-by: Leandro Awa <lawa@nvidia.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=104911
Fixes: 766c4cbfacd8 ("namei: d_is_negative() should be checked...")
Tested-by: Leandro Awa <lawa@nvidia.com>
Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
---
 fs/namei.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 726d211db484..33e9495a3129 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1558,8 +1558,6 @@ static int lookup_fast(struct nameidata *nd,
 		negative = d_is_negative(dentry);
 		if (read_seqcount_retry(&dentry->d_seq, seq))
 			return -ECHILD;
-		if (negative)
-			return -ENOENT;
 
 		/*
 		 * This sequence count validates that the parent had no
@@ -1580,6 +1578,12 @@ static int lookup_fast(struct nameidata *nd,
 				goto unlazy;
 			}
 		}
+		/*
+		 * Note: do negative dentry check after revalidation in
+		 * case that drops it.
+		 */
+		if (negative)
+			return -ENOENT;
 		path->mnt = mnt;
 		path->dentry = dentry;
 		if (likely(__follow_mount_rcu(nd, path, inode, seqp)))
-- 
2.4.3