linux-kernel - Re: /proc/diskstats buglet

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Message-ID: <CACVxJT_usd+8bDnctx6+4kgozE1GDONA+a+W+QWaa_tzuKCUEg@mail.gmail.com>
Date:	Tue, 26 Aug 2014 18:00:43 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Jens Axboe <axboe@...com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: /proc/diskstats buglet

On Tue, Aug 26, 2014 at 5:55 PM, Jens Axboe <axboe@...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
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