[PATCH - 2.6.15-rc5-mm3] Allow sysfs attribute files to be pollable.

[Neil Brown <neilb@suse.de>]

I suggested an early of this patch some time ago to see if it was an
acceptable approach and got zero feedback, which presumably means it
is perfect:-)

I've now reviewed it, fixed up the bits I didn't like, and tested it.
It works and I am happy with in.

So: I would like to submit it for inclusion in a future kernel.

Comments, or acks, please :-)

NeilBrown


---------
This allows an attribute file in sysfs to be polled for activity.

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.

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>

### Diffstat output
 ./drivers/md/md.c         |    1 
 ./fs/sysfs/dir.c          |    1 
 ./fs/sysfs/file.c         |   47 ++++++++++++++++++++++++++++++++++++++++++++++
 ./fs/sysfs/inode.c        |   20 +++++++++++++++++++
 ./fs/sysfs/sysfs.h        |    1 
 ./include/linux/kobject.h |    2 +
 ./include/linux/sysfs.h   |    7 ++++++
 ./lib/kobject.c           |    1 
 8 files changed, 80 insertions(+)

diff ./drivers/md/md.c~current~ ./drivers/md/md.c
--- ./drivers/md/md.c~current~	2005-12-21 09:42:07.000000000 +1100
+++ ./drivers/md/md.c	2005-12-21 09:43:52.000000000 +1100
@@ -162,6 +162,7 @@ void md_new_event(mddev_t *mddev)
 {
 	atomic_inc(&md_event_count);
 	wake_up(&md_event_waiters);
+	sysfs_notify(&mddev->kobj, NULL, "sync_action");
 }
 
 /*

diff ./fs/sysfs/dir.c~current~ ./fs/sysfs/dir.c
--- ./fs/sysfs/dir.c~current~	2005-12-21 09:43:51.000000000 +1100
+++ ./fs/sysfs/dir.c	2005-12-21 09:43:52.000000000 +1100
@@ -43,6 +43,7 @@ static struct sysfs_dirent * sysfs_new_d
 
 	memset(sd, 0, sizeof(*sd));
 	atomic_set(&sd->s_count, 1);
+	atomic_set(&sd->s_event, 0);
 	INIT_LIST_HEAD(&sd->s_children);
 	list_add(&sd->s_sibling, &parent_sd->s_children);
 	sd->s_element = element;

diff ./fs/sysfs/file.c~current~ ./fs/sysfs/file.c
--- ./fs/sysfs/file.c~current~	2005-12-21 09:43:51.000000000 +1100
+++ ./fs/sysfs/file.c	2005-12-21 09:44:28.000000000 +1100
@@ -7,6 +7,7 @@
 #include <linux/kobject.h>
 #include <linux/namei.h>
 #include <linux/limits.h>
+#include <linux/poll.h>
 
 #include <asm/uaccess.h>
 #include <asm/semaphore.h>
@@ -59,6 +60,7 @@ struct sysfs_buffer {
 	struct sysfs_ops	* ops;
 	struct semaphore	sem;
 	int			needs_read_fill;
+	int			event;
 };
 
 
@@ -74,6 +76,7 @@ struct sysfs_buffer {
  */
 static int fill_read_buffer(struct dentry * dentry, struct sysfs_buffer * buffer)
 {
+	struct sysfs_dirent * sd = dentry->d_fsdata;
 	struct attribute * attr = to_attr(dentry);
 	struct kobject * kobj = to_kobj(dentry->d_parent);
 	struct sysfs_ops * ops = buffer->ops;
@@ -85,6 +88,7 @@ static int fill_read_buffer(struct dentr
 	if (!buffer->page)
 		return -ENOMEM;
 
+	buffer->event = atomic_read(&sd->s_event);
 	count = ops->show(kobj,attr,buffer->page);
 	buffer->needs_read_fill = 0;
 	BUG_ON(count > (ssize_t)PAGE_SIZE);
@@ -357,12 +361,55 @@ static int sysfs_release(struct inode * 
 	return 0;
 }
 
+/* Sysfs attribute files are pollable.  The idea is that you read
+ * the content and then you use 'poll' or 'select' to wait for
+ * the content to change.  When the content changes (assuming the
+ * manager for the kobject supports notification), poll will
+ * return POLLERR|POLLPRI, and select will return the fd whether
+ * it is waiting for read, write, or exceptions.
+ * Once poll/select indicates that the value has changed, you
+ * need to close and re-open the file, as simply seeking and reading
+ * again will not get new data, or reset the state of 'poll'.
+ * Reminder: this only works for attributes which actively support
+ * it, and it is not possible to test an attribute from userspace
+ * to see if it supports poll.
+ */
+static unsigned int sysfs_poll(struct file *filp, poll_table *wait)
+{
+	struct sysfs_buffer * buffer = filp->private_data;
+	struct kobject * kobj = to_kobj(filp->f_dentry->d_parent);
+	struct sysfs_dirent * sd = filp->f_dentry->d_fsdata;
+	int res = 0;
+
+	poll_wait(filp, &kobj->poll, wait);
+
+	if (buffer->event != atomic_read(&sd->s_event))
+		res = POLLERR|POLLPRI;
+
+	return res;
+}
+
+void sysfs_notify(struct kobject * k, char *dir, char *attr)
+{
+	struct sysfs_dirent * sd = k->dentry->d_fsdata;
+	if (sd && dir)
+		sd = sysfs_find(sd, dir);
+	if (sd && attr)
+		sd = sysfs_find(sd, attr);
+	if (sd) {
+		atomic_inc(&sd->s_event);
+		wake_up_interruptible(&k->poll);
+	}
+}
+EXPORT_SYMBOL_GPL(sysfs_notify);
+
 struct file_operations sysfs_file_operations = {
 	.read		= sysfs_read_file,
 	.write		= sysfs_write_file,
 	.llseek		= generic_file_llseek,
 	.open		= sysfs_open_file,
 	.release	= sysfs_release,
+	.poll		= sysfs_poll,
 };
 
 

diff ./fs/sysfs/inode.c~current~ ./fs/sysfs/inode.c
--- ./fs/sysfs/inode.c~current~	2005-12-21 09:43:51.000000000 +1100
+++ ./fs/sysfs/inode.c	2005-12-21 09:43:52.000000000 +1100
@@ -247,3 +247,23 @@ void sysfs_hash_and_remove(struct dentry
 }
 
 
+struct sysfs_dirent *sysfs_find(struct sysfs_dirent *dir, const char * name)
+{
+	struct sysfs_dirent * sd, * rv = NULL;
+
+	if (dir->s_dentry == NULL ||
+	    dir->s_dentry->d_inode == NULL)
+		return NULL;
+
+	down(&dir->s_dentry->d_inode->i_sem);
+	list_for_each_entry(sd, &dir->s_children, s_sibling) {
+		if (!sd->s_element)
+			continue;
+		if (!strcmp(sysfs_get_name(sd), name)) {
+			rv = sd;
+			break;
+		}
+	}
+	up(&dir->s_dentry->d_inode->i_sem);
+	return rv;
+}

diff ./fs/sysfs/sysfs.h~current~ ./fs/sysfs/sysfs.h
--- ./fs/sysfs/sysfs.h~current~	2005-12-21 09:43:51.000000000 +1100
+++ ./fs/sysfs/sysfs.h	2005-12-21 09:43:52.000000000 +1100
@@ -10,6 +10,7 @@ extern int sysfs_make_dirent(struct sysf
 
 extern int sysfs_add_file(struct dentry *, const struct attribute *, int);
 extern void sysfs_hash_and_remove(struct dentry * dir, const char * name);
+extern struct sysfs_dirent *sysfs_find(struct sysfs_dirent *dir, const char * name);
 
 extern int sysfs_create_subdir(struct kobject *, const char *, struct dentry **);
 extern void sysfs_remove_subdir(struct dentry *);

diff ./include/linux/kobject.h~current~ ./include/linux/kobject.h
--- ./include/linux/kobject.h~current~	2005-12-21 09:43:51.000000000 +1100
+++ ./include/linux/kobject.h	2005-12-21 09:43:52.000000000 +1100
@@ -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
@@ -54,6 +55,7 @@ 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 ./include/linux/sysfs.h~current~ ./include/linux/sysfs.h
--- ./include/linux/sysfs.h~current~	2005-12-21 09:43:52.000000000 +1100
+++ ./include/linux/sysfs.h	2005-12-21 09:43:52.000000000 +1100
@@ -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
@@ -118,6 +119,7 @@ int sysfs_remove_bin_file(struct kobject
 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 */
 
 static inline int sysfs_create_dir(struct kobject * k)
@@ -185,6 +187,11 @@ static inline void sysfs_remove_group(st
 	;
 }
 
+static inline void sysfs_notify(struct kobject * k, char *dir, char *attr)
+{
+	;
+}
+
 #endif /* CONFIG_SYSFS */
 
 #endif /* _SYSFS_H_ */

diff ./lib/kobject.c~current~ ./lib/kobject.c
--- ./lib/kobject.c~current~	2005-12-21 09:43:52.000000000 +1100
+++ ./lib/kobject.c	2005-12-21 09:43:52.000000000 +1100
@@ -124,6 +124,7 @@ void kobject_init(struct kobject * kobj)
 {
 	kref_init(&kobj->kref);
 	INIT_LIST_HEAD(&kobj->entry);
+	init_waitqueue_head(&kobj->poll);
 	kobj->kset = kset_get(kobj->kset);
 }
 
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