Briefly looking at the code it seems like several global variables involved should be volatile: n_capabilities, enabled_capabilities, and capabilities.  Perhaps in a loop like in scheduleDoGC the compiler moves the reads of n_capabilites or capabilites outside the loop.  A failed requestSync in that loop would not get updated values for those global pointers.  That particular loop isn't doing that optimization for me, but I think it could happen without volatile.