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Re: kobject_uevent() and sysfs attribute sync

Hi Greg,

--- Greg KH <greg@xxxxxxxxx> escreveu:

> On Tue, Dec 05, 2006 at 08:57:48PM +0000, Mauricio Lin wrote:
> > Hi all,
> > 
> > I implemented a module that sends signals to user space using
> > kobject_uevent() when some conditions happen. That step is working.
> > 
> > When the signal is sent, a sysfs attribute is also changed (switch
> from
> > 0 to 1 or vice-versa). This means that the corresponding file under
> > /sys/ has its contents modified as well (using "_name"_show(struct
> > subsystem *subsys, char *page) function) .
> > 
> > The user space when receives the signal (sent by kobject_uevent())
> it
> > tries to read the file under /sys (that was changed to 1 when the
> > signal was sent), but the value is always 0. IMHO there is a sync
> > problem, because when user space gets the signal and it reads the
> file
> > under /sys, the value set to 1 (notify moment) could be changed
> back to
> > 0 (read moment) later.
> 
> Then userspace isn't getting there fast enough it seems :)

Yes, that's right.

> 
> > So is there any way to send a signal and guarantee that the
> attribute
> > will be read by user space before being changed by subsequent
> actions?
> > Is sysfs_notify() a possible solution for that?
> 
> How about just poll() on the sysfs file?  That way your program gets
> woken up, and you read the proper data.

Humm, is it efficient? It must be something that does not consume much
memory.

> 
> But what is the real thing you are trying to do here?  It seems you
> are
> trying to use sysfs in an odd way, perhaps you might want to do
> something else?

Humm, you gave me an idea. I have just to send the signal when the
attribute is changed to 1 and not 0, so there is no need to include an
additional read from user space to check if the value is 1 again.

Let me check the code again :)

Thanks,

Mauricio Lin.

> 
> thanks,
> 
> greg k-h
> 



	



	
		
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