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

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Date:	Thu, 16 Apr 2009 14:45:03 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Jens Axboe <jens.axboe@...cle.com>
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 Thursday 16 April 2009 13:05:57 Jens Axboe wrote:
> 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?

Sorry, not on current kernels. But on a very old 2.6.5 kernel.

Reading Documentation/iostats.txt and the changelog of commit 
e71bf0d0ee89e51b92776391c5634938236977d5 made me assume that this could be a 
problem even today.

> 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.

Isn't this also true for the stats protected by the part_stat_lock()? Only 
places where we are only reading seems to be called without the queue lock.

Thanks
Nikanth
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