devvga: better way to prevent blank hwaccel race

don't hold drawlock duing vga enable and disable, but just zero
the function pointers under drawlock *before* disabling the vga
device.

holding the drawlock while calling out into enable and disable
is not a good idea. with vgavesa, this might deadlock when
userspace realemu tries to print in a rio window with vgavesa.

1 files changed, 4 insertions, 9 deletions

diff --git a/sys/src/9/pc/devvga.c b/sys/src/9/pc/devvga.c
index 5d2ec72c8..5120f5946 100644
--- a/sys/src/9/pc/devvga.c
+++ b/sys/src/9/pc/devvga.c
@@ -309,23 +309,18 @@ vgactl(Cmdbuf *cb)
 		for(i = 0; vgadev[i]; i++){
 			if(strcmp(cb->f[1], vgadev[i]->name))
 				continue;
-			qlock(&drawlock);
-			if(waserror()){
-				qunlock(&drawlock);
-				nexterror();
-			}
 			if(scr->dev){
-				if(scr->dev->disable)
-					scr->dev->disable(scr);
+				qlock(&drawlock);
 				scr->fill = nil;
 				scr->scroll = nil;
 				scr->blank = nil;
+				qunlock(&drawlock);
+				if(scr->dev->disable)
+					scr->dev->disable(scr);
 			}
 			scr->dev = vgadev[i];
 			if(scr->dev->enable)
 				scr->dev->enable(scr);
-			qunlock(&drawlock);
-			poperror();
 			return;
 		}
 		break;