On 64-bit Linux condition-wait and condition-notify use 64 bit pointers as 32 bit unique futex values

[Ilya Perminov]

In Linux futex values are 32 bit. sb-futex version of conditon-wait/notify uses *current-thread* and a waitqueue as unique futex values. On a system with 64 bit pointers, only lower 32 bits are used possibly resulting in identical futex values and lost wakeups.

[Douglas Katzman (dougk)]

This problem could manifest only if 2 thread (or waitqueue) structures had addresses exactly 4 GB apart *and* were involved in an operation on the same futex word address at the same time.
So as a practical concern, it can't happen at all using the default heap size. I guess we'll have to map threads and waitqueues to "small" (32-bit) unique identifiers