Re: /proc/diskstats buglet

Jens Axboe
2014-08-26T15:01:34Z

On 08/26/2014 09:00 AM, Alexey Dobriyan wrote:
> On Tue, Aug 26, 2014 at 5:55 PM, Jens Axboe <axboe@fb.com> wrote:
>> On 08/26/2014 08:47 AM, Alexey Dobriyan wrote:
>>> Found and reproduced some time ago, almost forgot about :-)
>>>
>>> In part_round_stats_single(), ->stamp field is written but without
>>> locking SMP-wise.
>>>
>>> part->stamp = now;
>>>
>>> So, if two processes read /proc/diskstats, it is possible for "now -
>>> part->stamp" value to become negative.
>>>
>>> And indeed this can happen:
>>>
>>> now 4294755500, ->stamp 4294755501
>>> ------------[ cut here ]------------
>>> WARNING: CPU: 0 PID: 1950 at block/blk-core.c:1229
>>> part_round_stats_single+0xc0/0xd0()
>>> ...
>>> [<ffffffff811963f0>] part_round_stats_single+0xc0/0xd0
>>> [<ffffffff81196447>] part_round_stats+0x47/0x70
>>> [<ffffffff811a069d>] diskstats_show+0x8d/0x4b0
>>> ...
>>>
>>> Dunno how important used fields in /proc/diskstats but they can be
>>> clearly bogus.
>>
>> Easiest fix is probably just to do the now - part->stamp math earlier,
>> and ignore <= 0 instead of just now == part->stamp. I think that should
>> be good enough for disk stats, and (most importantly), it would avoid
>> the warning.
>>
>> Speaking of the warning, I don't see where that is. Where is it from?
>
> I inserted the warning to be sure bug exists and I am not misreading the code.

Ah got it, that makes sense. Care to send a patch to just ignore <= 0
now - part->stamp?

--
Jens Axboe