dm ioctl: support cookies for udev

Add support for passing a 32 bit "cookie" into the kernel with the
DM_SUSPEND, DM_DEV_RENAME and DM_DEV_REMOVE ioctls.  The (unsigned)
value of this cookie is returned to userspace alongside the uevents
issued by these ioctls in the variable DM_COOKIE.

This means the userspace process issuing these ioctls can be notified
by udev after udev has completed any actions triggered.

To minimise the interface extension, we pass the cookie into the
kernel in the event_nr field which is otherwise unused when calling
these ioctls.  Incrementing the version number allows userspace to
determine in advance whether or not the kernel supports the cookie.
If the kernel does support this but userspace does not, there should
be no impact as the new variable will just get ignored.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/include/linux/dm-ioctl.h b/include/linux/dm-ioctl.h
index 48e44ee2b466..2ab84c83c31a 100644
--- a/include/linux/dm-ioctl.h
+++ b/include/linux/dm-ioctl.h
@@ -123,6 +123,16 @@ struct dm_ioctl {
         __u32 target_count;     /* in/out */
         __s32 open_count;       /* out */
         __u32 flags;            /* in/out */
+
+        /*
+         * event_nr holds either the event number (input and output) or the
+         * udev cookie value (input only).
+         * The DM_DEV_WAIT ioctl takes an event number as input.
+         * The DM_SUSPEND, DM_DEV_REMOVE and DM_DEV_RENAME ioctls
+         * use the field as a cookie to return in the DM_COOKIE
+         * variable with the uevents they issue.
+         * For output, the ioctls return the event number, not the cookie.
+         */
         __u32 event_nr;         /* in/out */
         __u32 padding;
 
@@ -256,9 +266,9 @@ enum {
 #define DM_DEV_SET_GEOMETRY     _IOWR(DM_IOCTL, DM_DEV_SET_GEOMETRY_CMD, struct dm_ioctl)
 
 #define DM_VERSION_MAJOR        4
-#define DM_VERSION_MINOR        14
+#define DM_VERSION_MINOR        15
 #define DM_VERSION_PATCHLEVEL   0
-#define DM_VERSION_EXTRA        "-ioctl (2008-04-23)"
+#define DM_VERSION_EXTRA        "-ioctl (2009-04-01)"
 
 /* Status bits */
 #define DM_READONLY_FLAG        (1 << 0) /* In/Out */