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Message-ID: <53FC9FF5.7060106@fb.com> Date: Tue, 26 Aug 2014 08:55:49 -0600 From: Jens Axboe <axboe@...com> To: Alexey Dobriyan <adobriyan@...il.com> CC: Linux Kernel <linux-kernel@...r.kernel.org> Subject: Re: /proc/diskstats buglet 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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