Btrfs devel - Jan 2013 - [PATCH] Btrfs: fix freeing delayed ref head while still holding its mutex

I hit this error when reproducing a bug that would end in a transaction
abort.  We take the delayed ref head''s mutex to keep anybody from
processing
it while we''re destroying it, but we fail to drop the mutex before we
carry
on and free the damned thing.  Fix this by doing the remove logic for the
head ourselves and unlock the mutex, that way we can avoid use after
free''s
or hung tasks waiting on that mutex to come back so they know the delayed
ref completed.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
---
 fs/btrfs/disk-io.c |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 12a9547..51bff86 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3640,10 +3640,15 @@ int btrfs_destroy_delayed_refs(struct btrfs_transaction
*trans,
 			if (list_empty(&head->cluster))
 				delayed_refs->num_heads_ready--;
 			list_del_init(&head->cluster);
+			ref->in_tree = 0;
+			rb_erase(&ref->rb_node, &delayed_refs->root);
+			delayed_refs->num_entries--;
+			mutex_unlock(&head->mutex);
+		} else {
+			ref->in_tree = 0;
+			rb_erase(&ref->rb_node, &delayed_refs->root);
+			delayed_refs->num_entries--;
 		}
-		ref->in_tree = 0;
-		rb_erase(&ref->rb_node, &delayed_refs->root);
-		delayed_refs->num_entries--;
 
 		spin_unlock(&delayed_refs->lock);
 		btrfs_put_delayed_ref(ref);
-- 
1.7.7.6

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On Wed, Jan 30, 2013 at 04:06:18PM -0500, Josef Bacik
wrote:
> I hit this error when reproducing a bug that would end in a transaction
> abort.  We take the delayed ref head''s mutex to keep anybody from
processing
> it while we''re destroying it, but we fail to drop the mutex before
we carry
> on and free the damned thing.  Fix this by doing the remove logic for the
> head ourselves and unlock the mutex, that way we can avoid use after
free''s
> or hung tasks waiting on that mutex to come back so they know the delayed
> ref completed.  Thanks,
> 
> +			ref->in_tree = 0;
> +			rb_erase(&ref->rb_node, &delayed_refs->root);
> +			delayed_refs->num_entries--;
> +			mutex_unlock(&head->mutex);
> +		} else {
> +			ref->in_tree = 0;
> +			rb_erase(&ref->rb_node, &delayed_refs->root);
> +			delayed_refs->num_entries--;
Do you really need to duplicate the removal under the mutex?  Isn''t all
that protected by the delayed_refs->lock?

Isn''t it enough to just add the mutex_unlock()?

- z
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On Wed, Jan 30, 2013 at 03:16:35PM -0700, Zach Brown
wrote:
> On Wed, Jan 30, 2013 at 04:06:18PM -0500, Josef Bacik wrote:
> > I hit this error when reproducing a bug that would end in a
transaction
> > abort.  We take the delayed ref head''s mutex to keep anybody
from processing
> > it while we''re destroying it, but we fail to drop the mutex
before we carry
> > on and free the damned thing.  Fix this by doing the remove logic for
the
> > head ourselves and unlock the mutex, that way we can avoid use after
free''s
> > or hung tasks waiting on that mutex to come back so they know the
delayed
> > ref completed.  Thanks,
> > 
> 
> > +			ref->in_tree = 0;
> > +			rb_erase(&ref->rb_node, &delayed_refs->root);
> > +			delayed_refs->num_entries--;
> > +			mutex_unlock(&head->mutex);
> > +		} else {
> > +			ref->in_tree = 0;
> > +			rb_erase(&ref->rb_node, &delayed_refs->root);
> > +			delayed_refs->num_entries--;
> 
> Do you really need to duplicate the removal under the mutex? 
Isn''t all
> that protected by the delayed_refs->lock?
> 
> Isn''t it enough to just add the mutex_unlock()?
> 
Yeah I could re-arrange I guess, either way it''s not pretty but I guess
not
duplicating stuff is better.  Thanks,

Josef
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