Rethinking the futex API

>Waiting on multiple values can be done in user space. Association with a fd can be done with a userspace library writing a byte to a fifo on wakeup.

Wine already has this. The esync patchset uses eventfd to wait on multiple winapi mutexes [1]. But some Windows programs run out of file descriptors because they leak mutexes. File descriptors are a much more limited resource than memory. Also, the patch for wait for multiple futexes (called fsync in the wine community) has slightly better performance [2].

[1] https://github.com/zfigura/wine/blob/esync/README.esync
[2] https://github.com/ValveSoftware/Proton/issues/2966