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

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

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.

Before this patch my test box got into an infinite loop with fs_mark.  Now it
seems to run to completion.

-chris

diff -r 0b4ab489ffe1 transaction.c
--- a/transaction.c	Tue May 27 10:55:43 2008 -0400
+++ b/transaction.c	Sun Jun 08 22:23:50 2008 -0400
@@ -56,7 +56,6 @@ static noinline int join_transaction(str
 		total_trans++;
 		BUG_ON(!cur_trans);
 		root->fs_info->generation++;
-		root->fs_info->running_transaction = cur_trans;
 		root->fs_info->last_alloc = 0;
 		root->fs_info->last_data_alloc = 0;
 		cur_trans->num_writers = 1;
@@ -74,6 +73,9 @@ static noinline int join_transaction(str
 		extent_io_tree_init(&cur_trans->dirty_pages,
 				     root->fs_info->btree_inode->i_mapping,
 				     GFP_NOFS);
+		spin_lock(&root->fs_info->new_trans_lock);
+		root->fs_info->running_transaction = cur_trans;
+		spin_unlock(&root->fs_info->new_trans_lock);
 	} else {
 		cur_trans->num_writers++;
 		cur_trans->num_joined++;