Re: [RFC 0/5] fix races in CDC-WDM

[Oliver Neukum <oneukum@suse.com>]

Am Donnerstag, den 10.09.2020, 19:01 +0900 schrieb Tetsuo Handa:
> On 2020/09/10 18:09, Oliver Neukum wrote:
> > > Does it make sense to wait for response of someone else's usb_submit_urb() when
> > > someone is calling close(), for there is no guarantee that failure notice received
> > > via wdm_flush() via some file descriptor corresponds to usb_submit_urb() request from
> > > wdm_write() call from that file descriptor?
> > 
> > Well, user space may do multithreading. Whether it makes sense is
> > another question. We just need to return results confirming to the
> > standards. You noticed bugs. I think the next version will fix them.
> 
> My question is how do you handle if App1 and App2 (not multithreading but
> multiprocessing) shares the "desc" ? Unless

Well, device locking is a function of user space. This is Unix. If you
have two uncoordinated writers to a device, you cannot expect sensible
results. You can expect that the driver does not hang, of course.

> > > In patch "[RFC 3/5] CDC-WDM: making flush() interruptible", it is legal to return -EINTR
> > >  from close(). But I think that returning -EINTR from close() is not recommended because
> > > it can confuse multithreaded application (retrying close() upon -EINTR is not safe).
> > 
> > Well, but what is the alternative? Should we ignore signals?
> > 
> 
> we return the error from write() request (i.e. give up trying to report errors from
> close() event), we can't pass results to the intended recipients.

That means

* harming the single threaded for the sake of the few multithreaded
* it would not work for O_NONBLOCK
* if you use a device from multiple threads or tasks, locking is your
problem

Is there something we can do in flush()?

	Regards
		Oliver