linux-kernel - Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

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Message-ID: <20110105200825.GA11372@kroah.com>
Date:	Wed, 5 Jan 2011 12:08:25 -0800
From:	Greg KH <greg@...ah.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Jerome Marchand <jmarchan@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

On Wed, Jan 05, 2011 at 07:46:32PM +0100, Jens Axboe wrote:
> On 2011-01-05 16:58, Greg KH wrote:
> > On Wed, Jan 05, 2011 at 02:55:51PM +0100, Jens Axboe wrote:
> >> On 2011-01-04 22:00, Greg KH wrote:
> >>> On Tue, Jan 04, 2011 at 04:55:13PM +0100, Jerome Marchand wrote:
> >>>> Also add a refcount to struct hd_struct to keep the partition in
> >>>> memory as long as users exist. We use kref_test_and_get() to ensure
> >>>> we don't add a reference to a partition which is going away.
> >>>
> >>> No, don't do this, use a kref correctly and no such function should be
> >>> needed.
> >>>
> >>>> +	} else {
> >>>> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> >>>
> >>> That is the function that should properly increment the reference count
> >>> on the object.  If the object is "being removed", then it will return
> >>> NULL and you need to check that.  Do that and you do not need to add:
> >>
> >> It doesn't matter if you do it in there of after the fact, since the
> >> "lock" (RCU) is being held across the call. See my original suggestion
> >> here:
> >>
> >> https://lkml.org/lkml/2010/12/17/275
> > 
> > Ok, that's fine, just do it without adding that kref function and I have
> > no objection :)
> 
> Why? The code is perfectly fine. I originally objected to making an API
> like this for simple reference counting - seems I was right. Please
> actually look at the code and use. Alexey asked whether this was a toy
> API or a real one, I'd like to know that as well. If this is meant just
> for very basic get/put references, fine, then document that. But then
> what's the point of having this API in the first place?

The point is that you shouldn't have to roll your own reference count
code all over the place, 99% of the time, you should just use the
debugged, and documented, interface that the kernel provides with the
kref interface.

As for it being a "toy", it properly handles a very large majority of
the kernel reference counting logic today, in a race-free manner, so I
would not call that a "toy" at all.

Just use it properly.  As this patch series points out, adding this type
of function to the api is not a good idea, as it will be incorrect when
used.

thanks,

greg k-h
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