linux-kernel - Re: [PATCH 0/15] sysfs lazification final

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Message-ID: <m1y6lnlfgo.fsf@fess.ebiederm.org>
Date:	Mon, 30 Nov 2009 16:12:39 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Greg KH <greg@...ah.com>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>,
	Kay Sievers <kay.sievers@...y.org>,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-fsdevel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Benjamin LaHaise <bcrl@...et.ca>,
	Serge Hallyn <serue@...ibm.com>
Subject: Re: [PATCH 0/15] sysfs lazification final

Greg KH <greg@...ah.com> writes:

> On Mon, Nov 30, 2009 at 01:33:37PM -0800, Greg KH wrote:
>> On Sat, Nov 07, 2009 at 11:25:03PM -0800, Eric W. Biederman wrote:
>> > 
>> > The sysfs code updates the vfs caches immediately when the sysfs data
>> > structures change causing a lot of unnecessary complications.  The
>> > following patchset untangles that beast.  Allowing for simpler
>> > more straight forward code, the removal of a hack from the vfs
>> > to support sysfs, and human comprehensible locking on sysfs.
>> > 
>> > Most of these patches have already been reviewed and acked from the
>> > last time I had time to work on sysfs.
>> > 
>> > This acks have been folded in and the two small bugs found in the
>> > previous review have been fixed in the trailing patches (they are
>> > minor enough nits that even a bisect that happens to land in the
>> > middle should not see sysfs problems).
>> 
>> I've applied all of these to my tree now, and sorry, but something is
>> broken pretty badly.
>> 
>> When doing a simple 'ls /sys/class/input/' the process locks up.  This
>> means that X can't find any input devices, which makes for a bit of a
>> problem when wanting to use your mouse or keyboard :(
>> 
>> Attached is the state of my processes when this happens, if that helps
>> out any.
>> 
>> So I'm going to drop all of these from my tree again, as they are not
>> ready for merging at this point :(
>
> In looking at the stuck processes, it seems your last patch was the
> problem.  Removing that caused things to work again, so I've only
> dropped that one.
>
> Next time, please test your patches before submitting them :(

Weird I thought I had tested this.

That last patch to add locking that is only needed for vfs coherency
has certainly seen less testing than the others.

I also remember verify that nfs does the same thing, when in fact
nfs takes inode->i_lock not inode->i_mutex in the same situation.

generic_permission takes no locks so this is really about serializing
writes to the inode.  The vfs only takes inode->i_mutex, when calling
notify_change.

So it appears I have stepped into a murky corner of the vfs.  

I will take a look and do a bit more testing.   At the moment it looks
like a solution to serializing writes to the stat attributes on the inode is
going to be simply holding sysfs_mutex over inode_setattr in 
sysfs_setattr.  Assuming a solution is needed at all.

Eric
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