lguest: barrier me harder

Impact: barrier correctness in example launcher

I doubt either lguest user will complain about performance.

Reported-by: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index f2dbbf3bdeab..d36fcc0f2715 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -1630,6 +1630,13 @@ static bool service_io(struct device *dev)
                 }
         }
 
+        /* OK, so we noted that it was pretty poor to use an fdatasync as a
+         * barrier.  But Christoph Hellwig points out that we need a sync
+         * *afterwards* as well: "Barriers specify no reordering to the front
+         * or the back."  And Jens Axboe confirmed it, so here we are: */
+        if (out->type & VIRTIO_BLK_T_BARRIER)
+                fdatasync(vblk->fd);
+
         /* We can't trigger an IRQ, because we're not the Launcher.  It does
          * that when we tell it we're done. */
         add_used(dev->vq, head, wlen);