libdrm atomic_add_unless() may reverse return value meaning

Submitted by David Shao

Assigned to Default DRI bug account

Link to original bug (#100077)

Description

atomic_add_unless() in libdrm xf86atomic.h may reverse the meaning of its return value.

Linux kernel documentation seems to indicate something like: "Returns non-zero if @v was not @u, and zero otherwise."

A simple inverting the meaning of libdrm's return value allowed glxgears to properly function for a hacked version of pkgsrc on DragonFly 4.7-DEVELOPMENT on an Intel IvyBridge integrated graphics machine. glxgears was already properly functioning on the same machine for NetBSD current, NetBSD using its own atomic operations declared in its sys/atomic.h header.

A one line (character) fix is of the form:

--- xf86atomic.h.orig	2015-09-22 04:34:51.000000000 +0000
+++ xf86atomic.h
@@ -111,7 +111,7 @@ static inline int atomic_add_unless(atom
 	c = atomic_read(v);
 	while (c != unless && (old = atomic_cmpxchg(v, c, c + add)) != c)
 		c = old;
-	return c == unless;
+	return c != unless;
 }