[PATCH] sysfs: Allow sysfs attribute files to be pollable

It works like this:
  Open the file
  Read all the contents.
  Call poll requesting POLLERR or POLLPRI (so select/exceptfds works)
  When poll returns,
     close the file and go to top of loop.
   or lseek to start of file and go back to the 'read'.

Events are signaled by an object manager calling
   sysfs_notify(kobj, dir, attr);

If the dir is non-NULL, it is used to find a subdirectory which
contains the attribute (presumably created by sysfs_create_group).

This has a cost of one int  per attribute, one wait_queuehead per kobject,
one int per open file.

The name "sysfs_notify" may be confused with the inotify
functionality.  Maybe it would be nice to support inotify for sysfs
attributes as well?

This patch also uses sysfs_notify to allow /sys/block/md*/md/sync_action
to be pollable

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/kobject.h b/include/linux/kobject.h
index 4cb1214ec290..dcd0623be892 100644
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -24,6 +24,7 @@
 #include <linux/rwsem.h>
 #include <linux/kref.h>
 #include <linux/kernel.h>
+#include <linux/wait.h>
 #include <asm/atomic.h>
 
 #define KOBJ_NAME_LEN                   20
@@ -56,6 +57,7 @@ struct kobject {
         struct kset             * kset;
         struct kobj_type        * ktype;
         struct dentry           * dentry;
+        wait_queue_head_t       poll;
 };
 
 extern int kobject_set_name(struct kobject *, const char *, ...)
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index 392da5a6dacb..1ea5d3cda6ae 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -74,6 +74,7 @@ struct sysfs_dirent {
         umode_t                 s_mode;
         struct dentry           * s_dentry;
         struct iattr            * s_iattr;
+        atomic_t                s_event;
 };
 
 #define SYSFS_ROOT              0x0001
@@ -117,6 +118,7 @@ int sysfs_remove_bin_file(struct kobject * kobj, struct bin_attribute * attr);
 
 int sysfs_create_group(struct kobject *, const struct attribute_group *);
 void sysfs_remove_group(struct kobject *, const struct attribute_group *);
+void sysfs_notify(struct kobject * k, char *dir, char *attr);
 
 #else /* CONFIG_SYSFS */
 
@@ -185,6 +187,10 @@ static inline void sysfs_remove_group(struct kobject * k, const struct attribute
         ;
 }
 
+static inline void sysfs_notify(struct kobject * k, char *dir, char *attr)
+{
+}
+
 #endif /* CONFIG_SYSFS */
 
 #endif /* _SYSFS_H_ */