[Yaffs] YAFFS2 and kswapd dead lock problem

[Charles Manning]

On Saturday 03 December 2005 07:56, Lawson.Reed wrote:
> Found it.... took me 5 work days.
> And this deadlock issue IS in the YAFFS2 tip code on CVS here:
> http://www.aleph1.co.uk/cgi-bin/viewcvs.cgi/yaffs2/yaffs_fs.c?rev=1.34&view
>=auto
>

Thanks for this huge effort Reed.

I am quite surprised that someone else has not stumbled on this either.

Does it only impact on 2.4.x kernels or also 2.6?

> So, no one has seen this???
>
> Here is what is happening:
>
> Process 'A' grabs the YAFFS2 grossLock.
> Process 'B' preempts and it's job is to free unused inodes everywhere.
> (hint: 'B' is kswapd). So, 'B' sets I_FREEING. Then it calls
> yaffs_clear_inode() which needs the grossLock. So, it goes
> on the wait queue because 'A' has the grossLock.
>
> Now process 'A' runs. It's holding the grossLock. It calls
> yaffs_get_inode() which calls BACK UP to iget()... With
> the grossLock held! That calls find_inode(). It finds
> I_FREEING set and then gets put on a wait queue in
> __wait_on_freeing_inode().
>
> Presto chango deadlock.
>
> So, my solution is to make sure the grossLock is not held when
> calling yaffs_get_inode(). Plus, I added grossLocking to
> yaffs_read_inode() since NB's comment in there is no longer
> true.
>
> I ran my 20 thread torture test which usually deadlocks in under 30
> seconds. It ran overnight with this fix. The test found no compare
> errors in the 20 files that it reads and writes at random times with
> random data and random lengths.
>
> So, I strongly suggest that someone close to the YAFFSs effort review
> this change and incorporate it. I am kinda new to all this and I'm
> not even sure what the correct way to submit the changes are.
> So, let me know how I can help.

Did you mean to include some code?
The easiest is to send something in patch form using "diff -Naur old new", but 
others are fine too.

Thanx

Charles