[34-longterm 071/247] dm: dont take i_mutex to change device size

From: Paul Gortmaker
Date: Thu Jun 23 2011 - 14:24:33 EST

From: Mike Snitzer <snitzer@xxxxxxxxxx>

This is a commit scheduled for the next v2.6.34 longterm release.
If you see a problem with using this for longterm, please comment.

commit c217649bf2d60ac119afd71d938278cffd55962b upstream.

No longer needlessly hold md->bdev->bd_inode->i_mutex when changing the
size of a DM device. This additional locking is unnecessary because
i_size_write() is already protected by the existing critical section in
dm_swap_table(). DM already has a reference on md->bdev so the
associated bd_inode may be changed without lifetime concerns.

A negative side-effect of having held md->bdev->bd_inode->i_mutex was
that a concurrent DM device resize and flush (via fsync) would deadlock.
Dropping md->bdev->bd_inode->i_mutex eliminates this potential for
deadlock. The following reproducer no longer deadlocks:

Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: Alasdair G Kergon <agk@xxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
drivers/md/dm.c | 5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 56d192d..c955f7f 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2039,13 +2039,14 @@ static void event_callback(void *context)

+ * Protected by md->suspend_lock obtained by dm_swap_table().
+ */
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(md->bdev->bd_inode, (loff_t)size << SECTOR_SHIFT);
- mutex_unlock(&md->bdev->bd_inode->i_mutex);


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