Lock splice_read and splice_write functions

commit 1a25b1c4ce189e3926f2981f3302352a930086db
Author: Mikulas Patocka <mpatocka@redhat.com>
Date:   Mon Oct 15 17:20:17 2012 -0400

    Lock splice_read and splice_write functions
    
    Functions generic_file_splice_read and generic_file_splice_write access
    the pagecache directly. For block devices these functions must be locked
    so that block size is not changed while they are in progress.
    
    This patch is an additional fix for commit b87570f5d349 ("Fix a crash
    when block device is read and block size is changed at the same time")
    that locked aio_read, aio_write and mmap against block size change.
    
    Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Index: linux-3.6.x86_64/fs/block_dev.c
===================================================================
--- linux-3.6.x86_64.orig/fs/block_dev.c	2012-11-16 17:12:57.352936580 -0500
+++ linux-3.6.x86_64/fs/block_dev.c	2012-11-16 17:13:11.908887989 -0500
@@ -1662,6 +1662,39 @@
 	return ret;
 }
 
+static ssize_t blkdev_splice_read(struct file *file, loff_t *ppos,
+				  struct pipe_inode_info *pipe, size_t len,
+				  unsigned int flags)
+{
+	ssize_t ret;
+	struct block_device *bdev = I_BDEV(file->f_mapping->host);
+
+	percpu_down_read(&bdev->bd_block_size_semaphore);
+
+	ret = generic_file_splice_read(file, ppos, pipe, len, flags);
+
+	percpu_up_read(&bdev->bd_block_size_semaphore);
+
+	return ret;
+}
+
+static ssize_t blkdev_splice_write(struct pipe_inode_info *pipe,
+				   struct file *file, loff_t *ppos, size_t len,
+				   unsigned int flags)
+{
+	ssize_t ret;
+	struct block_device *bdev = I_BDEV(file->f_mapping->host);
+
+	percpu_down_read(&bdev->bd_block_size_semaphore);
+
+	ret = generic_file_splice_write(pipe, file, ppos, len, flags);
+
+	percpu_up_read(&bdev->bd_block_size_semaphore);
+
+	return ret;
+}
+
+
 /*
  * Try to release a page associated with block device when the system
  * is under memory pressure.
@@ -1700,8 +1733,8 @@
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	= compat_blkdev_ioctl,
 #endif
-	.splice_read	= generic_file_splice_read,
-	.splice_write	= generic_file_splice_write,
+	.splice_read	= blkdev_splice_read,
+	.splice_write	= blkdev_splice_write,
 };
 
 int ioctl_by_bdev(struct block_device *bdev, unsigned cmd, unsigned long arg)