devvga: fix race condition between writes to vgactl

to reproduce:

	for(i in `{seq 10}) echo softscreen off >/dev/vgactl

3 files changed, 15 insertions, 20 deletions

diff --git a/sys/src/9/pc/devvga.c b/sys/src/9/pc/devvga.c
index 07f2c407c..f52be3291 100644
--- a/sys/src/9/pc/devvga.c
+++ b/sys/src/9/pc/devvga.c
@@ -280,8 +280,10 @@ vgactl(VGAscr *scr, Cmdbuf *cb)
 			error("bad channel");
 		if(chantodepth(chan) != z)
 			error("depth, channel do not match");
+		qlock(&drawlock);
 		deletescreenimage();
 		setscreensize(scr, r.max.x, r.max.y, z, chan, scr->tilt);
+		qunlock(&drawlock);
 		return;
 
 	case CMactualsize:
@@ -296,6 +298,7 @@ vgactl(VGAscr *scr, Cmdbuf *cb)
 			error(Ebadarg);
 		if(r.max.x > scr->width || r.max.y > scr->height)
 			error("physical screen bigger than virtual");
+		qlock(&drawlock);
 		deletescreenimage();
 		setactualsize(scr, r);
 		goto Resized;
@@ -324,6 +327,7 @@ vgactl(VGAscr *scr, Cmdbuf *cb)
 		}
 		if(scr->gscreen == nil)
 			return;
+		qlock(&drawlock);
 		r = actualscreensize(scr);
 		chan = scr->gscreen->chan;
 		z = scr->gscreen->depth;
@@ -331,9 +335,11 @@ vgactl(VGAscr *scr, Cmdbuf *cb)
 		setscreensize(scr, scr->width, scr->height, z, chan, tilt);
 		setactualsize(scr, r);
 		/* no break */
-	case CMdrawinit:
-		if(scr->gscreen == nil)
+	Init:
+		if(scr->gscreen == nil){
+			qunlock(&drawlock);
 			error(Enoscreen);
+		}
 		if(scr->dev && scr->dev->drawinit)
 			scr->dev->drawinit(scr);
 		hwblank = scr->blank != nil;
@@ -341,8 +347,13 @@ vgactl(VGAscr *scr, Cmdbuf *cb)
 	Resized:
 		vgascreenwin(scr);
 		resetscreenimage();
+		qunlock(&drawlock);
 		return;
 
+	case CMdrawinit:
+		qlock(&drawlock);
+		goto Init;
+
 	case CMlinear:
 		if(cb->nf!=2 && cb->nf!=3)
 			error(Ebadarg);
diff --git a/sys/src/9/pc/screen.c b/sys/src/9/pc/screen.c
index 072cbb7c4..9ce091983 100644
--- a/sys/src/9/pc/screen.c
+++ b/sys/src/9/pc/screen.c
@@ -64,17 +64,11 @@ actualscreensize(VGAscr *scr)
 void
 setactualsize(VGAscr *scr, Rectangle r)
 {
-	qlock(&drawlock);
-
 	r.min = ZP;
 	r.max = tiltsize(scr->tilt, r.max);
-	if(rectclip(&r, scr->gscreen->r) == 0){
-		qunlock(&drawlock);
+	if(rectclip(&r, scr->gscreen->r) == 0)
 		return;
-	}
 	scr->gscreen->clipr = r;
-
-	qunlock(&drawlock);
 }
 
 static char*
@@ -136,11 +130,8 @@ setscreensize(VGAscr *scr, int x, int y, int z, ulong chan, int tilt)
 {
 	char *err;
 
-	qlock(&drawlock);
-	if(waserror()){
-		qunlock(&drawlock);
+	if(waserror())
 		nexterror();
-	}
 
 	if(memimageinit() < 0)
 		error("memimageinit failed");
@@ -169,7 +160,6 @@ setscreensize(VGAscr *scr, int x, int y, int z, ulong chan, int tilt)
 		cursoron();
 	}
 
-	qunlock(&drawlock);
 	poperror();
 }
 
diff --git a/sys/src/9/pc/vga.c b/sys/src/9/pc/vga.c
index 90682cd96..2150728f9 100644
--- a/sys/src/9/pc/vga.c
+++ b/sys/src/9/pc/vga.c
@@ -183,8 +183,6 @@ vgascreenwin(VGAscr* scr)
 	Point p;
 	int h;
 
-	qlock(&drawlock);
-	
 	h = scr->memdefont->height;
 	r = scr->gscreen->r;
 
@@ -215,11 +213,7 @@ vgascreenwin(VGAscr* scr)
 	curpos = window.min;
 
 	flushmemscreen(r);
-
-	qunlock(&drawlock);
-
 	vgascreenputs(kmesg.buf, kmesg.n);
-
 	screenputs = vgascreenputs;
 }