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Date:   Tue, 3 Sep 2019 13:37:19 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Qian Cai <cai@lca.pw>
Cc:     linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: "fs/namei.c: keep track of nd->root refcount status" causes boot
 panic
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On Tue, Sep 03, 2019 at 12:21:36AM -0400, Qian Cai wrote:
> The linux-next commit "fs/namei.c: keep track of nd->root refcount status” [1] causes boot panic on all
> architectures here on today’s linux-next (0902). Reverted it will fix the issue.

<swearing>

OK, I see what's going on.  Incremental to be folded in:

diff --git a/include/linux/namei.h b/include/linux/namei.h
index 20ce2f917ef4..2ed0942a67f8 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -20,8 +20,8 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_FOLLOW		0x0001	/* follow links at the end */
 #define LOOKUP_DIRECTORY	0x0002	/* require a directory */
 #define LOOKUP_AUTOMOUNT	0x0004  /* force terminal automount */
-#define LOOKUP_EMPTY		0x4000	/* accept empty path [user_... only] */
-#define LOOKUP_DOWN		0x8000	/* follow mounts in the starting point */
+#define LOOKUP_EMPTY		0x8000	/* accept empty path [user_... only] */
+#define LOOKUP_DOWN		0x10000	/* follow mounts in the starting point */
 
 #define LOOKUP_REVAL		0x0020	/* tell ->d_revalidate() to trust no cache */
 #define LOOKUP_RCU		0x0040	/* RCU pathwalk mode; semi-internal */