md/raid1: use md_bitmap_wait_behind_writes() in raid1_read_request()

Use the existed helper instead of open coding it to make the code cleaner.
There are no functional changes, and also avoid dereferencing bitmap
directly to prepare inventing a new bitmap.

Noted that this patch also export md_bitmap_wait_behind_writes(), which
is necessary for now, and the exported api will be removed in following
patches to convert bitmap apis into ops.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-2-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/md-bitmap.c

@@ -1851,6 +1851,7 @@ void md_bitmap_wait_behind_writes(struct mddev *mddev)
 			   atomic_read(&bitmap->behind_writes) == 0);
 	}
 }
+EXPORT_SYMBOL_GPL(md_bitmap_wait_behind_writes);
 
 void md_bitmap_destroy(struct mddev *mddev)
 {

drivers/md/raid1.c

@@ -1311,7 +1311,6 @@ static void raid1_read_request(struct mddev *mddev, struct bio *bio,
 	struct r1conf *conf = mddev->private;
 	struct raid1_info *mirror;
 	struct bio *read_bio;
-	struct bitmap *bitmap = mddev->bitmap;
 	const enum req_op op = bio_op(bio);
 	const blk_opf_t do_sync = bio->bi_opf & REQ_SYNC;
 	int max_sectors;
@@ -1364,15 +1363,13 @@ static void raid1_read_request(struct mddev *mddev, struct bio *bio,
 				    (unsigned long long)r1_bio->sector,
 				    mirror->rdev->bdev);
 
-	if (test_bit(WriteMostly, &mirror->rdev->flags) &&
-	    bitmap) {
+	if (test_bit(WriteMostly, &mirror->rdev->flags)) {
 		/*
 		 * Reading from a write-mostly device must take care not to
 		 * over-take any writes that are 'behind'
 		 */
 		mddev_add_trace_msg(mddev, "raid1 wait behind writes");
-		wait_event(bitmap->behind_wait,
-			   atomic_read(&bitmap->behind_writes) == 0);
+		md_bitmap_wait_behind_writes(mddev);
 	}
 
 	if (max_sectors < bio_sectors(bio)) {