Re: /proc/diskstats buglet

Alexey Dobriyan
2014-08-26T15:00:46Z

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.

Alexey