linux-kernel - Re: [Bluez-devel] Oops involving RFCOMM and sysfs

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Date:	Sat, 05 Jan 2008 23:30:25 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Gabor Gombas <gombasg@...aki.hu>,
	Dave Young <hidave.darkstar@...il.com>,
	linux-kernel@...r.kernel.org, bluez-devel@...ts.sf.net,
	Greg KH <greg@...ah.com>
Subject: Re: [Bluez-devel] Oops involving RFCOMM and sysfs

Hello.

Al Viro wrote:
> sysfs_get_dentry(),
>                 mutex_lock(&parent->d_inode->i_mutex);
> hitting parent->d_inode either NULL or very close to it, depending on your
> .config; most likely NULL, if offset of i_mutex is 0xb8 in your build.
> That's plausible - 0xb8 is what you'd get on UP build without spinlock
> debugging, lockdep, etc.
> 
> Assuming that this is what we get, everything looks explainable - we
> have sysfs_rename_dir() calling sysfs_get_dentry() while the parent
> gets evicted.  We don't have any exclusion, so while we are playing
> silly buggers with lookups in sysfs_get_dentry() we have parent become
> negative; the rest is obvious...

That part of code is walking down the sysfs tree from the s_root of
sysfs hierarchy and on each step parent is held using dget() while being
referenced, so I don't think they can turn negative there.

> AFAICS, the locking here is quite broken and frankly, sysfs_get_dentry()
> and the way it plays with fs/namei.c are ucking fugly.

Can you elaborate a bit?  The locking in sysfs is unconventional but
that's mostly from necessity.  It has dual interface - vfs and driver
model && vfs data structures (dentry and inode) are too big to always
keep around, so it basically becomes a small distributed file system
where the backing data can change asynchronously.

> Could you stick
> 	if (!parent->d_inode)
> 		printk(KERN_WARNING "sysfs locking blows: %s",
> 			parent->d_name.name);
> right before
>                 mutex_lock(&parent->d_inode->i_mutex);
>                 dentry = lookup_one_noperm(cur->s_name, parent);
>                 mutex_unlock(&parent->d_inode->i_mutex);
> in sysfs_get_dentry() (fs/sysfs/dir.c) and verify that it does, indeed,
> trigger?

Yes, please.

Thanks.

-- 
tejun
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