fs: Add the __data_racy annotation to backing_dev_info.ra_pages

Some but not all .ra_pages changes happen while block layer I/O is paused
with blk_mq_freeze_queue(). Filesystems may read .ra_pages even while
block layer I/O is paused, e.g. from inside their .fadvise callback.
Annotating all .ra_pages reads with READ_ONCE() would be cumbersome.
Hence, add the __data_racy annotatation to the .ra_pages member
variable.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: bvanassche

include/linux/backing-dev-defs.h

@@ -168,7 +168,9 @@ struct backing_dev_info {
 	u64 id;
 	struct rb_node rb_node; /* keyed by ->id */
 	struct list_head bdi_list;
-	unsigned long ra_pages;	/* max readahead in PAGE_SIZE units */
+	/* max readahead in PAGE_SIZE units */
+	unsigned long __data_racy ra_pages;
+
 	unsigned long io_pages;	/* max allowed IO size */
 
 	struct kref refcnt;	/* Reference counter for the structure */