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Re: [dm-devel] dm: use revalidate_disk to update device size after set_capacity

On Thu, Oct 28 2010 at  3:54pm -0400,
Mike Snitzer <snitzer redhat com> wrote:

> On Thu, Oct 28 2010 at  8:15am -0400,
> Jun'ichi Nomura <j-nomura ce jp nec com> wrote:

> > Though I can't point out actual problem,
> > I think it's deadlock-prone to take bd_mutex in dm_swap_table.
> > 
> > There are already codes which do I/O while holding bd_mutex,
> > e.g. block/ioctl.c, though the code is not called for dm,
> > so we can' just set a general rule "Don't do I/O while holding bd_mutex".
> Right, it would work provided we had that understanding within DM.

I think I misunderstood what you were saying as: no code does I/O to DM
while holding bd_mutex so we _can_ just set the general rule....

But re-reading your mail made me realize you were likely saying the
opposite?  e.g. your can' was meant to be can't.

So even though we currently can't see a real deadlock associated with
using ->bd_mutex directly you're contending there is potential for one
(may already be one in hiding or one could be introduced in the future).

That is fine, best to be conservative.

> I'll cleanup the patch from my previous mail to include some in-code
> comments and re-post it.  Please ack it if you agree that direct use of
> bd_mutex in __set_size() is a reasonable fix given the current code.

Taking a step back, don't we already have adequate locking for
__set_size's i_size_write() via dm_swap_table's md->suspend_lock?

So something like the following?

 drivers/md/dm.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 7cb1352..e71b8a1 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1996,9 +1996,8 @@ static void __set_size(struct mapped_device *md, sector_t size)
 	set_capacity(md->disk, size);
-	mutex_lock(&md->bdev->bd_inode->i_mutex);
+	/* i_size_write() is protected by md->suspend_lock */
 	i_size_write(md->bdev->bd_inode, (loff_t)size << SECTOR_SHIFT);
-	mutex_unlock(&md->bdev->bd_inode->i_mutex);

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