Re: [PATCH] usbnet: Fix two races between usbnet_stop() and the BH

From: Eugene Shatokhin
Date: Mon Aug 24 2015 - 13:01:07 EST

24.08.2015 16:29, BjÃrn Mork ÐÐÑÐÑ:
Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx> writes:

19.08.2015 15:31, BjÃrn Mork ÐÐÑÐÑ:
Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx> writes:

The problem is not in the reordering but rather in the fact that
"dev->flags = 0" is not necessarily atomic
w.r.t. "clear_bit(EVENT_RX_KILL, &dev->flags)", and vice versa.

So the following might be possible, although unlikely:

clear_bit: read dev->flags
clear_bit: clear EVENT_RX_KILL in the read value


clear_bit: write updated dev->flags

As a result, dev->flags may become non-zero again.

Ah, right. Thanks for explaining.

I cannot prove yet that this is an impossible situation. If anyone
can, please explain. If so, this part of the patch will not be needed.

I wonder if we could simply move the dev->flags = 0 down a few lines to
fix both issues? It doesn't seem to do anything useful except for
resetting the flags to a sane initial state after the device is down.

Stopping the tasklet rescheduling etc depends only on netif_running(),
which will be false when usbnet_stop is called. There is no need to
touch dev->flags for this to happen.

That was one of the first ideas we discussed here. Unfortunately, it
is probably not so simple.

Setting dev->flags to 0 makes some delayed operations do nothing and,
among other things, not to reschedule usbnet_bh().

Yes, but I believe that is merely a side effect. You should never need
to clear multiple flags to get the desired behaviour.

As you can see in drivers/net/usb/usbnet.c, usbnet_bh() can be called
as a tasklet function and as a timer function in a number of
situations (look for the usage of dev->bh and dev->delay there).

netif_running() is indeed false when usbnet_stop() runs, usbnet_stop()
also disables Tx. This seems to be enough for many cases where
usbnet_bh() is scheduled, but I am not so sure about the remaining
ones, namely:

1. A work function, usbnet_deferred_kevent(), may reschedule
usbnet_bh(). Looks like the workqueue is only stopped in
usbnet_disconnect(), so a work item might be processed while
usbnet_stop() works. Setting dev->flags to 0 makes the work function
do nothing, by the way. See also the comment in usbnet_stop() about

A work item may be placed to this workqueue in a number of ways, by
both usbnet module and the mini-drivers. It is not too easy to track
all these situations.

That's an understatement :)

2. rx_complete() and tx_complete() may schedule execution of
usbnet_bh() as a tasklet or a timer function. These two are URB
completion callbacks.

It seems, new Rx and Tx URBs cannot be submitted when usbnet_stop()
clears dev->flags, indeed. But it does not prevent the completion
handlers for the previously submitted URBs from running concurrently
with usbnet_stop(). The latter waits for them to complete (via
usbnet_terminate_urbs(dev)) but only if FLAG_AVOID_UNLINK_URBS is not
set in info->flags. rndis_wlan, however, sets this flag for a few
hardware models. So - no guarantees here as well.

FLAG_AVOID_UNLINK_URBS looks like it should be replaced by the newer
ability to keep the status urb active. I believe that must have been the
real reason for adding it, based on the commit message and the effect
the flag will have:

commit 1487cd5e76337555737cbc55d7d83f41460d198f
Author: Jussi Kivilinna <jussi.kivilinna@xxxxxxxx>
Date: Thu Jul 30 19:41:20 2009 +0300

usbnet: allow "minidriver" to prevent urb unlinking on usbnet_stop

rndis_wlan devices freeze after running usbnet_stop several times. It appears
that firmware freezes in state where it does not respond to any RNDIS commands
and device have to be physically unplugged/replugged. This patch lets
minidrivers to disable unlink_urbs on usbnet_stop through new info flag.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@xxxxxxxx>
Cc: David Brownell <dbrownell@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: John W. Linville <linville@xxxxxxxxxxxxx>

The rx urbs will not be resubmitted in any case, and there are of course
no tx urbs being submitted. So the only effect of this flag is on the
status/interrupt urb, which I can imagine some RNDIS devices wants
active all the time.

So FLAG_AVOID_UNLINK_URBS should probably be removed and replaced calls
to usbnet_status_start() and usbnet_status_stop(). This will require
testing on some of the devices with the original firmware problem

In any case: I do not think this flag should be considered when trying
to make usbnet_stop behaviour saner. It's only purpose is to
deliberately break usbnet_stop by not actually stopping.

If someone could list the particular bits of dev->flags that should be
cleared to make sure no deferred call could reschedule usbnet_bh(),
etc... Well, it would be enough to clear these first and use
dev->flags = 0 later, after tasklet_kill() and del_timer_sync(). I
cannot point out these particular bits now.

I don't think any of the flags must be cleared. The sequence

del_timer_sync (&dev->delay);
tasklet_kill (&dev->bh);

should prevent any rescheduling of usbnet_bh

If so, then, I suppose, one could ignore that FLAG_AVOID_UNLINK_URBS for now and just move dev->flags = 0 down as you suggested and as we thought before.

The patch will become simpler, indeed.

Besides, it is possible, although unlikely, that new event bits will
be added to dev->flags in the future. And one will need to keep track
of these to see if they should be cleared as well. I'd prever to play
safer for now and clear them all.

I don't think we should ever make a flag which will _have_ to be reset
for usbnet_stop. The only reason for clearing all flags is to reset the
state before the next open.

Yes, I see that we currently need to clear EVENT_DEV_OPEN in
usbnet_stop, but I really don't see what this flag gives us which isn't
already provided by netif_running(). It looks like a duplicate.


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