Re: /proc/diskstats buglet

[Jens Axboe <axboe@fb.com>]

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?

-- 
Jens Axboe