A copy-paste error in the configure patch adding POSIX named semaphore
support caused futex to be disabled when --{enable,disable}-semaphore
was not passed (i.e. on all non-NetBSD platforms). This caused
libxshmfence to fallback to pthread fences on Linux.
The X server and client applications need to use the same fence
implementation in order for DRI3 fences to work. If a GL client is
linked with a different libxshmfence built with futex support (which
is the usual case on Linux) while the server uses libxshmfence with
pthread, the client will hang while awaiting a fence when allocating
a buffer.
e6ad537986