linux-kernel - Re: [RFC 02/22] configfs: Add struct configfs_item_operations->check_link() in configfs

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Message-ID: <20100910194404.GJ885@mail.oracle.com>
Date:	Fri, 10 Sep 2010 12:44:04 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	James Bottomley <James.Bottomley@...e.de>,
	Jens Axboe <axboe@...nel.dk>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC 02/22] configfs: Add struct
 configfs_item_operations->check_link() in configfs_unlink()

On Fri, Sep 10, 2010 at 12:06:46PM -0700, Nicholas A. Bellinger wrote:
> On Fri, 2010-09-10 at 08:28 -0700, Joel Becker wrote:
> > 	The trivial solution is to refcount your ACLs.  You get both
> > allow_link() calls, so you should be able to increment a counter there,
> > and then drop them when the last drop_link() call is made.  That will
> > keep your consumer structures around until all links are exhausted.
> > 
> 
> So I am a bit confused wrt to this last response..  The ->check_link()
> patch and it's use in the fabric independent code within
> target_core_fabric_configfs.c does exactly this for the 'MappedLUN'
> symlink case, eg: requires the consumer to do the allow_link() +
> drop_link() refcounting, and add the
> API check into fs/configfs/symlink.c:configfs_unlink()

	You can refcount without check_link().
 
> Is there another form of configfs consumer refcounting that you had in
> mind beyond using an atomic_t for this with ->check_link() here..?

	I'm saying that you won't crash if you don't free the ACLs on
the first drop_link().  That is, the drop_link() goes through as
configfs wants it to, but you don't crash.

> So beyond a configfs consumer solution, what do you think about checking
> for the sub_child/group2/dst_0/dst_link style of symlink
> in fs/configfs/symlink.c:configfs_symlink() in order to add some form of
> internal refcount when the symlink source is within the same consumer
> LKM, but outside of the parent struct config_group..?
> 
> This would involve the conversion of fs/configfs/symlink.c:
> configfs_unlink() path to check for the existence of this internal
> refcount and returning -EPERM when any sub_child/group2/dst_0/dst_link
> exist when 'unlink sub_child/group1/src_0/src_link' is attempted.

	You're still fighting allowing the links to go away.  You
haven't explained why that is necessary.  You had a problem with a crash
because you expected one reference to your ACLs and actually have two,
but you can fix that without modifying configfs.

Joel

-- 

"I have never let my schooling interfere with my education."
        - Mark Twain

Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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