block: refator submit_bio_noacct

Split out a __submit_bio_noacct helper for the actual de-recursion
algorithm, and simplify the loop by using a continue when we can't
enter the queue for a bio.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1084,74 +1084,53 @@ static blk_qc_t __submit_bio(struct bio *bio)
 	return ret;
 }
 
-/**
- * submit_bio_noacct - re-submit a bio to the block device layer for I/O
- * @bio:  The bio describing the location in memory and on the device.
+/*
+ * The loop in this function may be a bit non-obvious, and so deserves some
+ * explanation:
  *
- * This is a version of submit_bio() that shall only be used for I/O that is
- * resubmitted to lower level drivers by stacking block drivers.  All file
- * systems and other upper level users of the block layer should use
- * submit_bio() instead.
+ *  - Before entering the loop, bio->bi_next is NULL (as all callers ensure
+ *    that), so we have a list with a single bio.
+ *  - We pretend that we have just taken it off a longer list, so we assign
+ *    bio_list to a pointer to the bio_list_on_stack, thus initialising the
+ *    bio_list of new bios to be added.  ->submit_bio() may indeed add some more
+ *    bios through a recursive call to submit_bio_noacct.  If it did, we find a
+ *    non-NULL value in bio_list and re-enter the loop from the top.
+ *  - In this case we really did just take the bio of the top of the list (no
+ *    pretending) and so remove it from bio_list, and call into ->submit_bio()
+ *    again.
+ *
+ * bio_list_on_stack[0] contains bios submitted by the current ->submit_bio.
+ * bio_list_on_stack[1] contains bios that were submitted before the current
+ *	->submit_bio_bio, but that haven't been processed yet.
  */
-blk_qc_t submit_bio_noacct(struct bio *bio)
+static blk_qc_t __submit_bio_noacct(struct bio *bio)
 {
-	/*
-	 * bio_list_on_stack[0] contains bios submitted by the current
-	 * ->submit_bio.
-	 * bio_list_on_stack[1] contains bios that were submitted before the
-	 * current ->submit_bio_bio, but that haven't been processed yet.
-	 */
 	struct bio_list bio_list_on_stack[2];
 	blk_qc_t ret = BLK_QC_T_NONE;
 
-	if (!submit_bio_checks(bio))
-		goto out;
-
-	/*
-	 * We only want one ->submit_bio to be active at a time, else
-	 * stack usage with stacked devices could be a problem.  So use
-	 * current->bio_list to keep a list of requests submited by a
-	 * ->submit_bio method.  current->bio_list is also used as a
-	 * flag to say if submit_bio_noacct is currently active in this
-	 * task or not.  If it is NULL, then no make_request is active.  If
-	 * it is non-NULL, then a make_request is active, and new requests
-	 * should be added at the tail
-	 */
-	if (current->bio_list) {
-		bio_list_add(&current->bio_list[0], bio);
-		goto out;
-	}
-
-	/* following loop may be a bit non-obvious, and so deserves some
-	 * explanation.
-	 * Before entering the loop, bio->bi_next is NULL (as all callers
-	 * ensure that) so we have a list with a single bio.
-	 * We pretend that we have just taken it off a longer list, so
-	 * we assign bio_list to a pointer to the bio_list_on_stack,
-	 * thus initialising the bio_list of new bios to be
-	 * added.  ->submit_bio() may indeed add some more bios
-	 * through a recursive call to submit_bio_noacct.  If it
-	 * did, we find a non-NULL value in bio_list and re-enter the loop
-	 * from the top.  In this case we really did just take the bio
-	 * of the top of the list (no pretending) and so remove it from
-	 * bio_list, and call into ->submit_bio() again.
-	 */
 	BUG_ON(bio->bi_next);
+
 	bio_list_init(&bio_list_on_stack[0]);
 	current->bio_list = bio_list_on_stack;
+
 	do {
 		struct request_queue *q = bio->bi_disk->queue;
-
-		if (likely(bio_queue_enter(bio) == 0)) {
 		struct bio_list lower, same;
 
-			/* Create a fresh bio_list for all subordinate requests */
+		if (unlikely(bio_queue_enter(bio) != 0))
+			continue;
+
+		/*
+		 * Create a fresh bio_list for all subordinate requests.
+		 */
 		bio_list_on_stack[1] = bio_list_on_stack[0];
 		bio_list_init(&bio_list_on_stack[0]);
+
 		ret = __submit_bio(bio);
 
-			/* sort new bios into those for a lower level
-			 * and those for the same level
+		/*
+		 * Sort new bios into those for a lower level and those for the
+		 * same level.
 		 */
 		bio_list_init(&lower);
 		bio_list_init(&same);
@@ -1160,18 +1139,46 @@ blk_qc_t submit_bio_noacct(struct bio *bio)
 				bio_list_add(&same, bio);
 			else
 				bio_list_add(&lower, bio);
-			/* now assemble so we handle the lowest level first */
+
+		/*
+		 * Now assemble so we handle the lowest level first.
+		 */
 		bio_list_merge(&bio_list_on_stack[0], &lower);
 		bio_list_merge(&bio_list_on_stack[0], &same);
 		bio_list_merge(&bio_list_on_stack[0], &bio_list_on_stack[1]);
-		}
-		bio = bio_list_pop(&bio_list_on_stack[0]);
-	} while (bio);
-	current->bio_list = NULL; /* deactivate */
+	} while ((bio = bio_list_pop(&bio_list_on_stack[0])));
 
-out:
+	current->bio_list = NULL;
 	return ret;
 }
+
+/**
+ * submit_bio_noacct - re-submit a bio to the block device layer for I/O
+ * @bio:  The bio describing the location in memory and on the device.
+ *
+ * This is a version of submit_bio() that shall only be used for I/O that is
+ * resubmitted to lower level drivers by stacking block drivers.  All file
+ * systems and other upper level users of the block layer should use
+ * submit_bio() instead.
+ */
+blk_qc_t submit_bio_noacct(struct bio *bio)
+{
+	if (!submit_bio_checks(bio))
+		return BLK_QC_T_NONE;
+
+	/*
+	 * We only want one ->submit_bio to be active at a time, else stack
+	 * usage with stacked devices could be a problem.  Use current->bio_list
+	 * to collect a list of requests submited by a ->submit_bio method while
+	 * it is active, and then process them after it returned.
+	 */
+	if (current->bio_list) {
+		bio_list_add(&current->bio_list[0], bio);
+		return BLK_QC_T_NONE;
+	}
+
+	return __submit_bio_noacct(bio);
+}
 EXPORT_SYMBOL(submit_bio_noacct);
 
 /**