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

From: Bjørn Mork <[email-masked]>
Date: Wed, 19 Aug 2015 12:54:29 +0200

Eugene Shatokhin <[email-masked]> writes:

> 19.08.2015 04:54, David Miller пишет:
>> From: Eugene Shatokhin <[email-masked]>
>> Date: Fri, 14 Aug 2015 19:58:36 +0300
>>
>>> 2. The second race is on dev->flags.
>>>
>>> dev->flags is set to 0 here:
>>> *0  usbnet_stop (usbnet.c:816)
>>>      /* deferred work (task, timer, softirq) must also stop.
>>>       * can't flush_scheduled_work() until we drop rtnl (later),
>>>       * else workers could deadlock; so make workers a NOP.
>>>       */
>>>      dev->flags = 0;
>>>      del_timer_sync (&dev->delay);
>>>      tasklet_kill (&dev->bh);
>>>
>>> And here, the code clears EVENT_RX_KILL bit in dev->flags, which may
>>> execute concurrently with the above operation:
>>> *0 clear_bit (bitops.h:113, inlined)
>>> *1 usbnet_bh (usbnet.c:1475)
>>>      /* restart RX again after disabling due to high error rate */
>>>      clear_bit(EVENT_RX_KILL, &dev->flags);
>>>
>>> It seems, setting dev->flags to 0 is not necessarily atomic w.r.t.
>>> clear_bit() and other bit operations with dev->flags. It is safer to
>>> make it atomic and this way, make the race harmless.
>>>
>>> While at it, the checking of EVENT_NO_RUNTIME_PM bit of dev->flags in
>>> usbnet_stop() was fixed too: the bit should be checked before dev->flags
>>> is cleared.
>>
>> The fix for this is excessive.
>>
>> Instead of all of this madness, looping over expensive clear_bit()
>> atomics, just do whatever it takes to make sure that usbnet_bh() is
>> quiesced and cannot execute any more.  Then you can safely clear
>> dev->flags normally.
>>
>
> If I understand it correctly, it is to make sure usbnet_bh() is not
> scheduled again that dev->flags should be set to 0 first, one way or
> another. That is what this madness is for.

Assuming there is a race which may reorder these, exactly what
difference does it make wrt EVENT_RX_KILL if you do

a)  clear_bit(EVENT_RX_KILL, &dev->flags);
    dev->flags = 0;

or

b)  dev->flags = 0;
    clear_bit(EVENT_RX_KILL, &dev->flags);

AFAICS, the result will be a cleared EVENT_RX_KILL bit in either case.

The EVENT_NO_RUNTIME_PM bug should definitely be fixed.  Please split
that out as a separate fix.  It's a separate issue, and should be
backported to all maintained stable releases it applies to (anything
from v3.8 and newer)

Bjørn