libdrm atomic_add_unless() may reverse return value meaning
Submitted by David Shao
Assigned to Default DRI bug account
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; }