sysctl: add support for poll()

Adding support for poll() in sysctl fs allows userspace to receive
notifications of changes in sysctl entries.  This adds a infrastructure to
allow files in sysctl fs to be pollable and implements it for hostname and
domainname.

[akpm@linux-foundation.org: s/declare/define/ for definitions]
Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
Cc: Greg KH <gregkh@suse.de>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 9a1ec10fd504..703cfa33a3ca 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -931,6 +931,7 @@ enum
 #ifdef __KERNEL__
 #include <linux/list.h>
 #include <linux/rcupdate.h>
+#include <linux/wait.h>
 
 /* For the /proc/sys support */
 struct ctl_table;
@@ -1011,6 +1012,26 @@ extern int proc_do_large_bitmap(struct ctl_table *, int,
  * cover common cases.
  */
 
+/* Support for userspace poll() to watch for changes */
+struct ctl_table_poll {
+        atomic_t event;
+        wait_queue_head_t wait;
+};
+
+static inline void *proc_sys_poll_event(struct ctl_table_poll *poll)
+{
+        return (void *)(unsigned long)atomic_read(&poll->event);
+}
+
+void proc_sys_poll_notify(struct ctl_table_poll *poll);
+
+#define __CTL_TABLE_POLL_INITIALIZER(name) {                            \
+        .event = ATOMIC_INIT(0),                                        \
+        .wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.wait) }
+
+#define DEFINE_CTL_TABLE_POLL(name)                                     \
+        struct ctl_table_poll name = __CTL_TABLE_POLL_INITIALIZER(name)
+
 /* A sysctl table is an array of struct ctl_table: */
 struct ctl_table 
 {
@@ -1021,6 +1042,7 @@ struct ctl_table
         struct ctl_table *child;
         struct ctl_table *parent;       /* Automatically set */
         proc_handler *proc_handler;     /* Callback for text formatting */
+        struct ctl_table_poll *poll;
         void *extra1;
         void *extra2;
 };