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

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Message-ID: <20110105162709.GA4506@kroah.com>
Date:	Wed, 5 Jan 2011 08:27:09 -0800
From:	Greg KH <greg@...ah.com>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, Jens Axboe <jaxboe@...ionio.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 05:19:19PM +0100, Jerome Marchand wrote:
> On 01/05/2011 05:00 PM, Greg KH wrote:
> > On Wed, Jan 05, 2011 at 02:51:28PM +0100, Jerome Marchand wrote:
> >> On 01/04/2011 10:00 PM, 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.
> >>
> >> Agreed.
> >>
> >>>  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:
> >>
> >> The object is actually removed in a rcu callback function. We could
> >> certainly add a flag to hd_struct, set by the release function, to
> >> indicate disk_map_sector_rcu() that the partition is being removed, but
> >> why not use the refcount instead?
> > 
> > Because you have to properly serialize the grabbing of a kref if you
> > don't have a valid pointer in the first place, otherwise it will not
> > work properly at all.  Your new function still does not properly handle
> > the race condition of dropping the last reference and then having the
> > kref be cleaned up.  You are giving false hope to the user of the api
> > that what they are doing is correct.
> > 
> 
> For clarification, is your objection only about not adding that misleading
> function to kref api (I understand that), or is my code actually racy?

As you are adding a misleading function to the kref api, and by using
it, causing a racy implementation, I would say both :)

thanks,

greg k-h
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