It came to my attention while reviewing some proposals for additional error checking in musl libc that the kernel folks introduced a quiet regression in the futex ABI for robust mutexes. Prior to kernel commit 9c40365a65d62d7c06a95fb331b3442cb02d2fd9, bit 29 of the futex lock word was reserved, meaning it could never be part of a TID. This allowed both glibc and musl to use special values like (glibc): /* Magic cookie representing robust mutex with dead owner. */ #define PTHREAD_MUTEX_INCONSISTENT INT_MAX /* Magic cookie representing not recoverable robust mutex. */ #define PTHREAD_MUTEX_NOTRECOVERABLE (INT_MAX - 1) to represent special states needed for robust mutex consistency handling, without the risk that, when masked with FUTEX_TID_MASK (0x3fffffff), they could be equal to the TID of a real task, which could result in the kernel robustlist-processing mishandling them. In practice this is only a documentation regression. The real limit on TIDs is well below that, at something like 22-bit last I checked. However to be future-proof, and as other systems may implement the Linux API/ABI (e.g. things like WSL1, FreeBSD Linux syscall layer, etc.), I believe it's important that the documented interface be compatible with the way it actually needs to be used here. This means either bit 29 should have its reserved status restored, or there should be a guarantee that the values 0x3fffffff and 0x3ffffffe are reserved and compare not-equal to any actual TID. Depending on how folks want to proceed with this, I can propose a patch to the documentation, or leave that to others. Rich
