ARC: Remove b_cv from struct l1arc_buf_hdr

Earlier as part of #14123 I've removed one use of b_cv.  This patch
reuses the same approach to remove the other one from much more
rare code path.

This saves 16 bytes of L1 ARC header on FreeBSD (reducing it from
200 to 184 bytes) and seems even more on Linux.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15340

include/sys/arc_impl.h

@@ -159,8 +159,6 @@ struct arc_write_callback {
  * these two allocation states.
  */
 typedef struct l1arc_buf_hdr {
-	/* for waiting on reads to complete */
-	kcondvar_t		b_cv;
 	uint8_t			b_byteswap;
 
 	/* protected by arc state mutex */

module/zfs/arc.c

@@ -1151,7 +1151,6 @@ hdr_full_cons(void *vbuf, void *unused, int kmflag)
 
 	memset(hdr, 0, HDR_FULL_SIZE);
 	hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
-	cv_init(&hdr->b_l1hdr.b_cv, NULL, CV_DEFAULT, NULL);
 	zfs_refcount_create(&hdr->b_l1hdr.b_refcnt);
 #ifdef ZFS_DEBUG
 	mutex_init(&hdr->b_l1hdr.b_freeze_lock, NULL, MUTEX_DEFAULT, NULL);
@@ -1211,7 +1210,6 @@ hdr_full_dest(void *vbuf, void *unused)
 	arc_buf_hdr_t *hdr = vbuf;
 
 	ASSERT(HDR_EMPTY(hdr));
-	cv_destroy(&hdr->b_l1hdr.b_cv);
 	zfs_refcount_destroy(&hdr->b_l1hdr.b_refcnt);
 #ifdef ZFS_DEBUG
 	mutex_destroy(&hdr->b_l1hdr.b_freeze_lock);
@@ -5586,13 +5584,6 @@ arc_read_done(zio_t *zio)
 			buf_hash_remove(hdr);
 	}
 
-	/*
-	 * Broadcast before we drop the hash_lock to avoid the possibility
-	 * that the hdr (and hence the cv) might be freed before we get to
-	 * the cv_broadcast().
-	 */
-	cv_broadcast(&hdr->b_l1hdr.b_cv);
-
 	arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
 	(void) remove_reference(hdr, hdr);
 
@@ -5787,8 +5778,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 				}
 				acb->acb_zio_head = head_zio;
 				acb->acb_next = hdr->b_l1hdr.b_acb;
-				if (hdr->b_l1hdr.b_acb)
-					hdr->b_l1hdr.b_acb->acb_prev = acb;
+				hdr->b_l1hdr.b_acb->acb_prev = acb;
 				hdr->b_l1hdr.b_acb = acb;
 			}
 			mutex_exit(hash_lock);
@@ -5928,8 +5918,28 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 				 * and so the performance impact shouldn't
 				 * matter.
 				 */
-				cv_wait(&hdr->b_l1hdr.b_cv, hash_lock);
+				arc_callback_t *acb = kmem_zalloc(
+				    sizeof (arc_callback_t), KM_SLEEP);
+				acb->acb_wait = B_TRUE;
+				mutex_init(&acb->acb_wait_lock, NULL,
+				    MUTEX_DEFAULT, NULL);
+				cv_init(&acb->acb_wait_cv, NULL, CV_DEFAULT,
+				    NULL);
+				acb->acb_zio_head =
+				    hdr->b_l1hdr.b_acb->acb_zio_head;
+				acb->acb_next = hdr->b_l1hdr.b_acb;
+				hdr->b_l1hdr.b_acb->acb_prev = acb;
+				hdr->b_l1hdr.b_acb = acb;
 				mutex_exit(hash_lock);
+				mutex_enter(&acb->acb_wait_lock);
+				while (acb->acb_wait) {
+					cv_wait(&acb->acb_wait_cv,
+					    &acb->acb_wait_lock);
+				}
+				mutex_exit(&acb->acb_wait_lock);
+				mutex_destroy(&acb->acb_wait_lock);
+				cv_destroy(&acb->acb_wait_cv);
+				kmem_free(acb, sizeof (arc_callback_t));
 				goto top;
 			}
 		}