/proc/diskstats buglet

[Alexey Dobriyan]

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.

    Alexey
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