direct-io: don't read inode->i_blkbits multiple times

Since directio can work on a raw block device, and the block size of the
device can change under it, we need to do the same thing that
fs/buffer.c now does: read the block size a single time, using
ACCESS_ONCE().

Reading it multiple times can get different results, which will then
confuse the code because it actually encodes the i_blksize in
relationship to the underlying logical blocksize.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/direct-io.c

@@ -540,6 +540,7 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
 	sector_t fs_endblk;	/* Into file, in filesystem-sized blocks */
 	unsigned long fs_count;	/* Number of filesystem-sized blocks */
 	int create;
+	unsigned int i_blkbits = sdio->blkbits + sdio->blkfactor;
 
 	/*
 	 * If there was a memory error and we've overwritten all the
@@ -554,7 +555,7 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
 		fs_count = fs_endblk - fs_startblk + 1;
 
 		map_bh->b_state = 0;
-		map_bh->b_size = fs_count << dio->inode->i_blkbits;
+		map_bh->b_size = fs_count << i_blkbits;
 
 		/*
 		 * For writes inside i_size on a DIO_SKIP_HOLES filesystem we
@@ -1053,7 +1054,8 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	int seg;
 	size_t size;
 	unsigned long addr;
-	unsigned blkbits = inode->i_blkbits;
+	unsigned i_blkbits = ACCESS_ONCE(inode->i_blkbits);
+	unsigned blkbits = i_blkbits;
 	unsigned blocksize_mask = (1 << blkbits) - 1;
 	ssize_t retval = -EINVAL;
 	loff_t end = offset;
@@ -1149,7 +1151,7 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	dio->inode = inode;
 	dio->rw = rw;
 	sdio.blkbits = blkbits;
-	sdio.blkfactor = inode->i_blkbits - blkbits;
+	sdio.blkfactor = i_blkbits - blkbits;
 	sdio.block_in_file = offset >> blkbits;
 
 	sdio.get_block = get_block;