[PATCH] [77/275] dm: dont take i_mutex to change device size

[Andi Kleen]

2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
From: Mike Snitzer <snitzer@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: Alasdair G Kergon <agk@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

---
 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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