linux-kernel - Re: [RFD driver-core] Lifetime problems of the current driver model

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Date:	Mon, 2 Apr 2007 02:33:24 -0700
From:	Greg KH <gregkh@...e.de>
To:	Tejun Heo <htejun@...il.com>
Cc:	James Bottomley <James.Bottomley@...elEye.com>, hugh@...itas.com,
	cornelia.huck@...ibm.com, dmitry.torokhov@...il.com,
	oneukum@...e.de, maneesh@...ibm.com, rpurdie@...ys.net,
	Jeff Garzik <jgarzik@...ox.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	SCSI Mailing List <linux-scsi@...r.kernel.org>
Subject: Re: [RFD driver-core] Lifetime problems of the current driver model

On Mon, Apr 02, 2007 at 04:59:43AM +0900, Tejun Heo wrote:
> 
> Dependencies between sysfs/kobject objects are clearer now and I think
> I got the locking and referencing correct.  This patch immediately
> fixes 'sysfs attr grabbing the wrong kobject and module' problem -
> sysfs and module lifetimes are unrelated now.  We can drop
> half-working attr->owner.
> 
> * A sysfs node no longer hold reference to its kobject.  It just
>   attaches itself to the kobject on creation and detaches on deletion.
> 
> * For a dir node, sysfs_dirent no longer directly points to kobject.
>   It points to sysfs_dir which contains pointer to kobject and a rwsem
>   to protect it.
> 
> * An open file doesn't hold a reference to kobject.  It holds a
>   reference to sysfs_dirent.  kobject pointer is verified and
>   show/store are performed with rwsem read-locked.  Deletion
>   disconnects the sysfs from its kobject while the rwsem is
>   write-locked.  This mechanism replaces buffer orphaning and kobject
>   validation during open.

Ah, very nice.

> * attr ops is determined on sysfs node creation not on open.  This is
>   a step toward making kobject opaque in sysfs.
> 
> * bin files are handled similarly but mmap makes the open file hold
>   reference to the kobject till it's closed.  Any better ideas?

This is probably needed, and might be acceptable, and I think we can
live with it as there is only a very small number of sysfs files that
fall into this category.

> * symlink is reimplemented in terms of sysfs_dirents instead of
>   kobjects.  sysfs_dirent->s_parent is added and each sysfs_dirent
>   holds reference to its parent.  Currently walking up the tree
>   requires read locking and unlocking sysfs_dir at each level.  This
>   can be removed if name is added to sysfs_dirent.
> 
> * As kobject can be disconnected anytime and sysfs code still needs to
>   look follow dentry link in kobject, kobject->dentry is protected by
>   dcache_lock.  Once kobject becomes opaque to sysfs, this hack can go
>   away.  All in all, making kobject completely opaque in sysfs isn't
>   too far away after this patch although it would require mass code
>   update.

What would need to be updated after this?

> What do you think about this approach?  If it's acceptable, I'll test
> further and split the patch into logical steps to get it reviewed
> better.

At first glance, I think it looks fine, but Maneesh is the one who
understands this code better than anyone, so I would like to get his
opinion on it.

I think you should start splitting it all up into steps, which will help
us review it a whole lot easier, as overall, I think this is a very
worthy goal.

thanks so much for doing this work,

greg k-h
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