linux-kernel - [PATCHv1 2/2] xenfs: replace xenbus and privcmd with symlinks

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Date:	Tue, 17 May 2016 11:27:23 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Alexander Viro <viro@...iv.linux.org.uk>
CC:	David Vrabel <david.vrabel@...rix.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xenproject.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	Juergen Gross <jgross@...e.com>
Subject: [PATCHv1 2/2] xenfs: replace xenbus and privcmd with symlinks

/proc/xen/xenbus does not work correctly.  A read blocked waiting for
a xenstore message holds the mutex needed for atomic file position
updates.  This blocks any writes on the same file handle, which can
deadlock if the write is needed to unblock the read.

/proc/xen/xenbus is supposed to be identical to the character device
/dev/xen/xenbus so replace the file with a symlink.

Similarly, replace /proc/xen/privcmd with a symlink since it should be
the same as /dev/xen/privcmd.

Signed-off-by: David Vrabel <david.vrabel@...rix.com>
---
v2:
- remove unneeded includes
---
 drivers/xen/xenfs/super.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/xen/xenfs/super.c b/drivers/xen/xenfs/super.c
index 8559a71..0f2e2cd 100644
--- a/drivers/xen/xenfs/super.c
+++ b/drivers/xen/xenfs/super.c
@@ -18,8 +18,6 @@
 #include <xen/xen.h>
 
 #include "xenfs.h"
-#include "../privcmd.h"
-#include "../xenbus/xenbus_comms.h"
 
 #include <asm/xen/hypervisor.h>
 
@@ -45,16 +43,16 @@ static const struct file_operations capabilities_file_ops = {
 static int xenfs_fill_super(struct super_block *sb, void *data, int silent)
 {
 	static struct tree_descr xenfs_files[] = {
-		[2] = { "xenbus", &xen_xenbus_fops, S_IRUSR|S_IWUSR },
+		[2] = { "xenbus", NULL, S_IFLNK | S_IRWXUGO, "/dev/xen/xenbus" },
 		{ "capabilities", &capabilities_file_ops, S_IRUGO },
-		{ "privcmd", &xen_privcmd_fops, S_IRUSR|S_IWUSR },
+		{ "privcmd", NULL, S_IFLNK | S_IRWXUGO, "/dev/xen/privcmd" },
 		{""},
 	};
 
 	static struct tree_descr xenfs_init_files[] = {
-		[2] = { "xenbus", &xen_xenbus_fops, S_IRUSR|S_IWUSR },
+		[2] = { "xenbus", NULL, S_IFLNK | S_IRWXUGO, "/dev/xen/xenbus" },
 		{ "capabilities", &capabilities_file_ops, S_IRUGO },
-		{ "privcmd", &xen_privcmd_fops, S_IRUSR|S_IWUSR },
+		{ "privcmd", NULL, S_IFLNK | S_IRWXUGO, "/dev/xen/privcmd" },
 		{ "xsd_kva", &xsd_kva_file_ops, S_IRUSR|S_IWUSR},
 		{ "xsd_port", &xsd_port_file_ops, S_IRUSR|S_IWUSR},
 #ifdef CONFIG_XEN_SYMS
-- 
2.1.4