Re: btrfs panic - BUG: soft lockup - CPU#0 stuck for 61s! [fs_mark:4573]

[Chris Mason <chris.mason@oracle.com>]

On Mon, 2008-06-09 at 17:10 -0700, Mingming Cao wrote:
> On Sun, 2008-06-08 at 22:37 -0400, Chris Mason wrote:
> > On Thu, 05 Jun 2008 13:43:48 -0400
> > Ric Wheeler <rwheeler@redhat.com> wrote:
> > 
> > > Chris Mason wrote:
> > > > On Mon, Jun 02, 2008 at 01:52:47PM -0400, Ric Wheeler wrote:
> > > >   
> > > >> I can reliably get btrfs to panic by running my fs_mark code on a
> > > >> newly created file system with lots of threads on an 8-way box. If
> > > >> this is too aggressive, let me know ;-)
> > > >>
> > > >> Here is a summary of the panic:
> > > >>     
> > > >
> > > > BTW, exactly how are you running fs_mark?  Mingming reminded me that
> > > > strictly speaking this patch shouldn't be required, so there might
> > > > be other related problems.
> > > >
> > > > -chris
> > > >
> > > >   
> > > It still crashes, Mingming is clearly correct ;-)
> > > 
> > 
> > Grin, I never should have doubted her.
> > 
> :) 
> 
> > So, the actual fix should be below.  It looks like the problem is that I've got
> > a race in setting the pointer to a new transaction, which makes the
> > data=ordered code take a spin lock that hasn't yet been setup.
> > 
> 
> Just to be clear, so the data=ordered code(btrfs_del_ordered_inode())
> takes a spin lock (new_trans_lock) and assume the new transaction has
> been setup, that races with join_transaction resetting the current
> running transaction()? 
> 
Yes

> I also see the btrfs_commit_transaction() could reset the
> root->fs_info->running_transaction to be NULL, but we did not check NULL
> pointer in the data=ordered mode code, is this a potential Bug? Or it is
> covered somewhere else?
> 

Thanks for double checking these.

We don't check it in btrfs_add_ordered_inode because that must be called
with the transaction running.

btrfs_ordered_throttle is safe because it doesn't actually deref the
pointer, it just checks for changes to it.  The important part of
ordered_throttle is the writeback count.

So, the others should be safe, but please let me know if you see any
holes there.

-chris