Re: Bug with USB proc_bulk in 2.4 kernel and possibly bug in proc_ioctl in 2.6

[Pete Zaitcev <zaitcev@redhat.com>]

On Fri, 7 Jul 2006 17:24:56 -0700, Benjamin Cherian <benjamin.cherian.kernel@gmail.com> wrote:

> I believe that there is a bug in the proc_bulk function in drivers/usb/devio.c 
> in the current 2.4 kernel. In the 2.4.28 proc_ioctl was changed so that the 
> device would be locked before proc_bulk was called (See line 1275 of 
> devio.c). In 2.4.27, no locking occurred. However, this leads to problems if 
> one thread is continuously attempting to read and another one needs to write 
> because the write thread can never access the device. []

The decision to cripple the devio in this way was conscious. The problem
scenario with 2.4.27 and earlier was how many devices would fail if
a control and other transfer were submitted simultaneously. In other
words, when desktop components (e.g. HAL) rescanned /proc/bus/usb/devices,
some other devices would throw errors. The most notorious example
would be TEAC CD-210PU, because of how widespread and popular it is.

In other words, I crippled your application for the sake of a bunch
of users of other devices. You have to realize though, that your
concerns weren't voiced in the time and it was widely believed that
user-mode drivers did not submit several URBs at once (primarily
because the queueing in HCDs of 2.4 was a suspect).

> I would have submitted a patch, but it seems like the locking/unlocking 
> mechanism is different in 2.4 and 2.6 and I wasn't sure if I would break 
> other stuff.

If you do send a patch, you need to account for the serialization
somehow. The linux-usb-devel people considered a few ideas, such
as per-device blacklists and locking flags, backport of string caching
from 2.6 (to alleviate the most common case of /proc/usb/usb/devices).

But it seems to me that the best approach would be if you made
a special case for bulk+bulk and locked out control transfers somehow.

Another option is to look at an in-kernel driver architecture for
your device. What is it, anyway?

-- Pete
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