Re: /proc/diskstats buglet

[Alexey Dobriyan <adobriyan@gmail.com>]

On Tue, Aug 26, 2014 at 09:01:28AM -0600, Jens Axboe wrote:
> 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?

It will be still possible for 2 contexts to execute part_round_stats_single()
simultaneously. Should lead to double accounting?

	Alexey