Question about the usage of pthread_mutex_t's __data.__lock

[Martin Galvan]

Thanks a lot for your answer. Just out of curiosity: looking at the
code in __pthread_mutex_lock_full, it says:

"The PI support requires the Linux futex system call.  If that's not
available, pthread_mutex_init should never have allowed the type to be
set.  So it will get the default case for an invalid type".

This sounds like there's a way to block threads without having the
__NR_futex syscall. However, I followed the code all the way from the
LLL_MUTEX_LOCK macro in nptl/pthread_mutex_lock.c to the part where a
thread will block on a mutex and I saw we always end up using the
futex syscall. From what I saw, it's always something like:

LLL_MUTEX_LOCK -> lll_lock (may be arch-dependant) -> __lll_lock_wait
-> lll_futex_wait -> INTERNAL_SYSCALL

INTERNAL_SYSCALL seems to be an arch-dependant macro which ends up
calling __NR_futex. There may be a couple of intermediate steps in
some archs (such as x86), but it seems to me that we'll always need
the syscall to block threads. Am I missing something?