sysfs: refill attribute buffer when reading from offset 0

Requiring userspace to close and re-open sysfs attributes has been the
policy since before 2.6.12.  It allows userspace to get a consistent
snapshot of kernel state and consume it with incremental reads and seeks.

Now, if the file position is zero the kernel assumes userspace wants to see
the new value.  The application for this change is to allow a userspace
RAID metadata handler to check the state of an array without causing any
memory allocations.  Thus not causing writeback to a raid array that might
be blocked waiting for userspace to take action.

Cc: Neil Brown <neilb@suse.de>
Acked-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/filesystems/sysfs.txt

@@ -176,8 +176,10 @@ implementations:
   Recall that an attribute should only be exporting one value, or an
   array of similar values, so this shouldn't be that expensive. 
 
-  This allows userspace to do partial reads and seeks arbitrarily over
-  the entire file at will. 
+  This allows userspace to do partial reads and forward seeks
+  arbitrarily over the entire file at will. If userspace seeks back to
+  zero or does a pread(2) with an offset of '0' the show() method will
+  be called again, rearmed, to fill the buffer.
 
 - On write(2), sysfs expects the entire buffer to be passed during the
   first write. Sysfs then passes the entire buffer to the store()
@@ -192,6 +194,9 @@ implementations:
 
 Other notes:
 
+- Writing causes the show() method to be rearmed regardless of current
+  file position.
+
 - The buffer will always be PAGE_SIZE bytes in length. On i386, this
   is 4096. 
 

fs/sysfs/file.c

@@ -129,7 +129,7 @@ sysfs_read_file(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 	ssize_t retval = 0;
 
 	mutex_lock(&buffer->mutex);
-	if (buffer->needs_read_fill) {
+	if (buffer->needs_read_fill || *ppos == 0) {
 		retval = fill_read_buffer(file->f_path.dentry,buffer);
 		if (retval)
 			goto out;
@@ -410,8 +410,7 @@ static int sysfs_release(struct inode *inode, struct file *filp)
  * 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'.
+ * need to close and re-open the file, or seek to 0 and read again.
  * 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 (Neither 'poll' nor 'select' return