[PATCH 1/4] Add a lock that will be needed by the next patch

[Mikulas Patocka <mpatocka@redhat.com>]

Add a lock that will be needed by the next patch.

I am submitting this as a separate patch, because the act of adding an
extra field to "struct block_device" affects performance significantly.

So that performance change from adding the lock field can be
differentiated from the performance change of locking.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/char/raw.c |    2 -
 fs/block_dev.c     |   60 +++++++++++++++++++++++++++++++++++++++++++++++++++--
 include/linux/fs.h |    4 +++
 3 files changed, 63 insertions(+), 3 deletions(-)

Index: linux-3.5-rc6-devel/include/linux/fs.h
===================================================================
--- linux-3.5-rc6-devel.orig/include/linux/fs.h	2012-07-16 20:20:12.000000000 +0200
+++ linux-3.5-rc6-devel/include/linux/fs.h	2012-07-16 01:29:21.000000000 +0200
@@ -713,6 +713,8 @@ struct block_device {
 	int			bd_fsfreeze_count;
 	/* Mutex for freeze */
 	struct mutex		bd_fsfreeze_mutex;
+	/* A semaphore that prevents I/O while block size is being changed */
+	struct rw_semaphore	bd_block_size_semaphore;
 };
 
 /*