linux-kernel - [PATCHv4 2/3] xenfs: replace xenbus and privcmd with symlinks

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Message-ID: <1477669959-9486-3-git-send-email-david.vrabel@citrix.com>
Date:   Fri, 28 Oct 2016 16:52:38 +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: [PATCHv4 2/3] 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>
---
v4:
- Make xen_xenbus_fops and xen_privcmd_fops static.
---
 drivers/xen/privcmd.c                    |  5 +----
 drivers/xen/privcmd.h                    |  3 ---
 drivers/xen/xenbus/xenbus_comms.h        |  2 --
 drivers/xen/xenbus/xenbus_dev_frontend.c |  3 +--
 drivers/xen/xenfs/super.c                | 10 ++++------
 5 files changed, 6 insertions(+), 17 deletions(-)
 delete mode 100644 drivers/xen/privcmd.h

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 702040f..12ece8d 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -37,8 +37,6 @@
 #include <xen/xen-ops.h>
 #include <xen/balloon.h>
 
-#include "privcmd.h"
-
 MODULE_LICENSE("GPL");
 
 #define PRIV_VMA_LOCKED ((void *)1)
@@ -644,12 +642,11 @@ static int privcmd_vma_range_is_mapped(
 				   is_mapped_fn, NULL) != 0;
 }
 
-const struct file_operations xen_privcmd_fops = {
+const static struct file_operations xen_privcmd_fops = {
 	.owner = THIS_MODULE,
 	.unlocked_ioctl = privcmd_ioctl,
 	.mmap = privcmd_mmap,
 };
-EXPORT_SYMBOL_GPL(xen_privcmd_fops);
 
 static struct miscdevice privcmd_dev = {
 	.minor = MISC_DYNAMIC_MINOR,
diff --git a/drivers/xen/privcmd.h b/drivers/xen/privcmd.h
deleted file mode 100644
index 14facae..0000000
--- a/drivers/xen/privcmd.h
+++ /dev/null
@@ -1,3 +0,0 @@
-#include <linux/fs.h>
-
-extern const struct file_operations xen_privcmd_fops;
diff --git a/drivers/xen/xenbus/xenbus_comms.h b/drivers/xen/xenbus/xenbus_comms.h
index e74f9c1..39efb85 100644
--- a/drivers/xen/xenbus/xenbus_comms.h
+++ b/drivers/xen/xenbus/xenbus_comms.h
@@ -47,6 +47,4 @@ extern struct xenstore_domain_interface *xen_store_interface;
 extern int xen_store_evtchn;
 extern enum xenstore_init xen_store_domain_type;
 
-extern const struct file_operations xen_xenbus_fops;
-
 #endif /* _XENBUS_COMMS_H */
diff --git a/drivers/xen/xenbus/xenbus_dev_frontend.c b/drivers/xen/xenbus/xenbus_dev_frontend.c
index c1010f01..a45e7f2 100644
--- a/drivers/xen/xenbus/xenbus_dev_frontend.c
+++ b/drivers/xen/xenbus/xenbus_dev_frontend.c
@@ -598,7 +598,7 @@ static unsigned int xenbus_file_poll(struct file *file, poll_table *wait)
 	return 0;
 }
 
-const struct file_operations xen_xenbus_fops = {
+const static struct file_operations xen_xenbus_fops = {
 	.read = xenbus_file_read,
 	.write = xenbus_file_write,
 	.open = xenbus_file_open,
@@ -606,7 +606,6 @@ const struct file_operations xen_xenbus_fops = {
 	.poll = xenbus_file_poll,
 	.llseek = no_llseek,
 };
-EXPORT_SYMBOL_GPL(xen_xenbus_fops);
 
 static struct miscdevice xenbus_dev = {
 	.minor = MISC_DYNAMIC_MINOR,
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