Add bioq_takefirst().

If the bioq is empty, NULL is returned.  Otherwise the front element
is removed and returned.

This can simplify locking in many drivers from:

	lock()
	bp = bioq_first(bq);
	if (bp == NULL) {
		unlock()
		return
	}
	bioq_remove(bp, bq)
	unlock
to:
	lock()
	bp = bioq_takefirst(bq);
	unlock()
	if (bp == NULL)
		return;

2 files changed, 13 insertions, 6 deletions

diff --git a/sys/kern/subr_disk.c b/sys/kern/subr_disk.c
index 9756ca6..fcd0c9d 100644
--- a/sys/kern/subr_disk.c
+++ b/sys/kern/subr_disk.c
@@ -94,13 +94,8 @@ bioq_flush(struct bio_queue_head *head, struct devstat *stp, int error)
 {
 	struct bio *bp;
 
-	for (;;) {
-		bp = bioq_first(head);
-		if (bp == NULL)
-			break;
-		bioq_remove(head, bp);
+	while ((bp = bioq_takefirst(head)) != NULL)
 		biofinish(bp, stp, error);
-	}
 }
 
 void
@@ -117,6 +112,16 @@ bioq_first(struct bio_queue_head *head)
 	return (TAILQ_FIRST(&head->queue));
 }
 
+struct bio *
+bioq_takefirst(struct bio_queue_head *head)
+{
+	struct bio *bp;
+
+	bp = TAILQ_FIRST(&head->queue);
+	if (bp != NULL)
+		bioq_remove(head, bp);
+	return (bp);
+}
 
 /*
  * Seek sort for disks.
diff --git a/sys/sys/bio.h b/sys/sys/bio.h
index 33ae4e7..0f42cff 100644
--- a/sys/sys/bio.h
+++ b/sys/sys/bio.h
@@ -88,6 +88,7 @@ struct bio {
 #define BIO_WRITE	0x02
 #define BIO_DELETE	0x04
 #define BIO_GETATTR	0x08
+#define BIO_CMD0	0x20	/* Available for local hacks */
 #define BIO_CMD1	0x40	/* Available for local hacks */
 #define BIO_CMD2	0x80	/* Available for local hacks */
 
@@ -113,6 +114,7 @@ int biowait(struct bio *bp, const char *wchan);
 
 void bioq_disksort(struct bio_queue_head *ap, struct bio *bp);
 struct bio *bioq_first(struct bio_queue_head *head);
+struct bio *bioq_takefirst(struct bio_queue_head *head);
 void bioq_flush(struct bio_queue_head *head, struct devstat *stp, int error);
 void bioq_init(struct bio_queue_head *head);
 void bioq_insert_tail(struct bio_queue_head *head, struct bio *bp);