On 08/26/2014 09:00 AM, Alexey Dobriyan wrote: > On Tue, Aug 26, 2014 at 5:55 PM, Jens Axboe <[email protected]> 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? -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
