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Subject[PATCH] [77/275] dm: dont take i_mutex to change device size
Date
2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
From: Mike Snitzer <snitzer@redhat.com>

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:
https://www.redhat.com/archives/dm-devel/2009-July/msg00284.html

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>

---
drivers/md/dm.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

Index: linux-2.6.35.y/drivers/md/dm.c
===================================================================
--- linux-2.6.35.y.orig/drivers/md/dm.c 2011-03-29 22:51:39.284717971 -0700
+++ linux-2.6.35.y/drivers/md/dm.c 2011-03-29 23:02:59.762306252 -0700
@@ -2040,13 +2040,14 @@
wake_up(&md->eventq);
}

+/*
+ * 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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