Re: /proc/diskstats buglet
From: Jens Axboe
Date: Tue Aug 26 2014 - 11:01:37 EST
On 08/26/2014 09:00 AM, Alexey Dobriyan wrote:
> On Tue, Aug 26, 2014 at 5:55 PM, Jens Axboe <axboe@xxxxxx> 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
>>> [<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?
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