Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx> writes:
> 19.08.2015 04:54, David Miller пишет:
>> From: Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx>
>> 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
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