https://bugs.freedesktop.org/show_bug.cgi?id=100077
Bug ID: 100077 Summary: libdrm atomic_add_unless() may reverse return value meaning Product: DRI Version: unspecified Hardware: x86-64 (AMD64) OS: BSD (Others) Status: NEW Severity: normal Priority: medium Component: libdrm Assignee: dri-devel@lists.freedesktop.org Reporter: davshao@gmail.com
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; }