linux-kernel - Re: [PATCH] [RFC] make hd_struct->in_flight atomic to avoid diskstat corruption

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Date:	Thu, 16 Apr 2009 09:35:57 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nikanth Karthikesan <knikanth@...e.de>
Cc:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] [RFC] make hd_struct->in_flight atomic to avoid
	diskstat corruption

On Thu, Apr 16 2009, Nikanth Karthikesan wrote:
> The disk statistics exported to userspace through proc and sysfs are
> not protected by locks to avoid performance overhead. Since most of
> the statistics are maintained in the per_cpu struct disk_stats, the
> chances of them getting corrupted is negligible. But the in_flight
> counter, that records the no of requests currently in progress is not
> per-cpu. This increases the chance of it getting corrupted. And
> corruption of this value would result in visibly distorted statistics
> such as negative in_flight. This can be avoided by making this field
> atomic.

Hmm. Did you observe this behaviour? A quick glance at the code reveals
that the callers of part_inc_in_flight() and part_dec_in_flight() in the
block layer are always done under the queue lock. Ditto
part_round_stats(), which calls part_round_stats_single() and also needs
protection for in_flight.

That basically just leaves the code reading this out and reporting, and
driver calls to part_round_stats(). I'd suggest looking there instead,
we're not going to make ->in_flight an atomic just because of some
silliness there that could be fixed.

-- 
Jens Axboe

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