Fix synchronous behavior in __vdev_disk_physio()

Commit b39c22b set the READ_SYNC and WRITE_SYNC flags for a bio
based on the ZIO_PRIORITY_* flag passed in.  This had the unnoticed
side-effect of making the vdev_disk_io_start() synchronous for
certain I/Os.

This in turn resulted in vdev_disk_io_start() being able to
re-dispatch zio's which would result in a RCU stalls when a disk
was removed from the system.  Additionally, this could negatively
impact performance and may explain the performance regressions
reported in both openzfs#3829 and openzfs#3780.

This patch resolves the issue by making the blocking behavior
dependant on a 'wait' flag being passed rather than overloading
the passed bio flags.

Finally, the WRITE_SYNC and READ_SYNC behavior is restricted to
non-rotational devices where there is no benefit to queuing to
aggregate the I/O.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue openzfs#3780
Issue openzfs#3829
Issue openzfs#3652

config/kernel.m4

@@ -25,9 +25,6 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_BIO_END_IO_T_ARGS
 	ZFS_AC_KERNEL_BIO_RW_BARRIER
 	ZFS_AC_KERNEL_BIO_RW_DISCARD
-	ZFS_AC_KERNEL_BIO_RW_SYNC
-	ZFS_AC_KERNEL_BIO_RW_SYNCIO
-	ZFS_AC_KERNEL_REQ_SYNC
 	ZFS_AC_KERNEL_BLK_QUEUE_FLUSH
 	ZFS_AC_KERNEL_BLK_QUEUE_MAX_HW_SECTORS
 	ZFS_AC_KERNEL_BLK_QUEUE_MAX_SEGMENTS

module/zfs/vdev_disk.c

@@ -369,27 +369,6 @@ vdev_disk_dio_free(dio_request_t *dr)
 	    sizeof (struct bio *) * dr->dr_bio_count);
 }
 
-static int
-vdev_disk_dio_is_sync(dio_request_t *dr)
-{
-#ifdef HAVE_BIO_RW_SYNC
-	/* BIO_RW_SYNC preferred interface from 2.6.12-2.6.29 */
-	return (dr->dr_rw & (1 << BIO_RW_SYNC));
-#else
-#ifdef HAVE_BIO_RW_SYNCIO
-	/* BIO_RW_SYNCIO preferred interface from 2.6.30-2.6.35 */
-	return (dr->dr_rw & (1 << BIO_RW_SYNCIO));
-#else
-#ifdef HAVE_REQ_SYNC
-	/* REQ_SYNC preferred interface from 2.6.36-2.6.xx */
-	return (dr->dr_rw & REQ_SYNC);
-#else
-#error "Unable to determine bio sync flag"
-#endif /* HAVE_REQ_SYNC */
-#endif /* HAVE_BIO_RW_SYNC */
-#endif /* HAVE_BIO_RW_SYNCIO */
-}
-
 static void
 vdev_disk_dio_get(dio_request_t *dr)
 {
@@ -444,7 +423,7 @@ BIO_END_IO_PROTO(vdev_disk_physio_completion, bio, size, error)
 	rc = vdev_disk_dio_put(dr);
 
 	/* Wake up synchronous waiter this is the last outstanding bio */
-	if ((rc == 1) && vdev_disk_dio_is_sync(dr))
+	if (rc == 1)
 		complete(&dr->dr_comp);
 
 	BIO_END_IO_RETURN(0);
@@ -514,7 +493,7 @@ vdev_submit_bio(int rw, struct bio *bio)
 
 static int
 __vdev_disk_physio(struct block_device *bdev, zio_t *zio, caddr_t kbuf_ptr,
-    size_t kbuf_size, uint64_t kbuf_offset, int flags)
+    size_t kbuf_size, uint64_t kbuf_offset, int flags, int wait)
 {
 	dio_request_t *dr;
 	caddr_t bio_ptr;
@@ -605,7 +584,7 @@ __vdev_disk_physio(struct block_device *bdev, zio_t *zio, caddr_t kbuf_ptr,
 	 * only synchronous consumer is vdev_disk_read_rootlabel() all other
 	 * IO originating from vdev_disk_io_start() is asynchronous.
 	 */
-	if (vdev_disk_dio_is_sync(dr)) {
+	if (wait) {
 		wait_for_completion(&dr->dr_comp);
 		error = dr->dr_error;
 		ASSERT3S(atomic_read(&dr->dr_ref), ==, 1);
@@ -621,7 +600,7 @@ vdev_disk_physio(struct block_device *bdev, caddr_t kbuf,
     size_t size, uint64_t offset, int flags)
 {
 	bio_set_flags_failfast(bdev, &flags);
-	return (__vdev_disk_physio(bdev, NULL, kbuf, size, offset, flags));
+	return (__vdev_disk_physio(bdev, NULL, kbuf, size, offset, flags, 1));
 }
 
 BIO_END_IO_PROTO(vdev_disk_io_flush_completion, bio, size, rc)
@@ -672,6 +651,7 @@ vdev_disk_io_start(zio_t *zio)
 {
 	vdev_t *v = zio->io_vd;
 	vdev_disk_t *vd = v->vdev_tsd;
+	zio_priority_t pri = zio->io_priority;
 	int flags, error;
 
 	switch (zio->io_type) {
@@ -711,14 +691,14 @@ vdev_disk_io_start(zio_t *zio)
 		zio_execute(zio);
 		return;
 	case ZIO_TYPE_WRITE:
-		if (zio->io_priority == ZIO_PRIORITY_SYNC_WRITE)
+		if ((pri == ZIO_PRIORITY_SYNC_WRITE) && (v->vdev_nonrot))
 			flags = WRITE_SYNC;
 		else
 			flags = WRITE;
 		break;
 
 	case ZIO_TYPE_READ:
-		if (zio->io_priority == ZIO_PRIORITY_SYNC_READ)
+		if ((pri == ZIO_PRIORITY_SYNC_READ) && (v->vdev_nonrot))
 			flags = READ_SYNC;
 		else
 			flags = READ;
@@ -731,7 +711,7 @@ vdev_disk_io_start(zio_t *zio)
 	}
 
 	error = __vdev_disk_physio(vd->vd_bdev, zio, zio->io_data,
-	    zio->io_size, zio->io_offset, flags);
+	    zio->io_size, zio->io_offset, flags, 0);
 	if (error) {
 		zio->io_error = error;
 		zio_interrupt(zio);