Subject: Re: [PATCH] usbnet: Fix two races between usbnet_stop() and the BH
To: David Miller <davem@davemloft.net>
References: <55CE1D7E.2070400@rosalab.ru>
 <1439571516-11862-1-git-send-email-eugene.shatokhin@rosalab.ru>
 <20150818.185407.1667358232705414236.davem@davemloft.net>
Cc: oneukum@suse.com, netdev@vger.kernel.org, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org
From: Eugene Shatokhin <eugene.shatokhin@rosalab.ru>
Organization: ROSA
Message-ID: <55D436D5.6010105@rosalab.ru>
Date: Wed, 19 Aug 2015 10:57:09 +0300
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19.08.2015 04:54, David Miller пишет:
> From: Eugene Shatokhin <eugene.shatokhin@rosalab.ru>
> 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.

tasklet_kill() will wait then for the already running instance of 
usbnet_bh() (if one is running). After that, it is guaranteed BH is not 
running and will not be re-scheduled.

As for the performance concerns, I doubt that usbnet_stop() is anywhere 
on the critical path. I have been testing this patch for some time and 
haven't seen any new performance issues with it yet.

If needed, it is possible to measure and compare the time needed for 
usbnet_stop() before and after this patch and try to estimate the impact 
of this on the overall performance.

Regards,
Eugene

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