Re: /proc/diskstats buglet
From: Alexey Dobriyan
Date: Tue Aug 26 2014 - 11:00:50 EST
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.
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