19.08.2015 13:54, Bjørn Mork пишет:
Eugene Shatokhin <eugene.shatok...@rosalab.ru> writes:
19.08.2015 04:54, David Miller пишет:
From: Eugene Shatokhin <eugene.shatok...@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.
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.
Thanks for the review!
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:
CPU0 CPU1
clear_bit: read dev->flags
clear_bit: clear EVENT_RX_KILL in the read value
dev->flags=0;
clear_bit: write updated dev->flags
As a result, dev->flags may become non-zero again.
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.
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)
Yes, that makes sense. However, this fix was originally provided by
Oliver Neukum rather than me, so I would like to hear his opinion as
well first.
Bjørn
Regards,
Eugene
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