how to check if pthread_mutex is based on robust futex

Question

I am trying to use robust futex based pthread mutex in Linux because I need to be both fast and robust (recover the "dead" lock). How can I check if the pthread mutex library on any Linux system is based off robust futex?

Thanks!

Answer 1

If you have the futex(2) system call and if it is used (just strace(1) a 10 line application that uses mutexes) then you have the robust feature as the futex(2) system call only entered the kernel after robustness has been built into it. This does not mean that you are using robust futexes, just that you have the feature in the kernel.

Next you want to know that your libc supports it. Any version above 2.9 supports it. Just check your version.

If you are writing a multi-threaded application then you don't really need the robustness of the futexes since you control the threads and can make sure that threads release the mutexes they use before they die or register a cleanup function to do the lock releasing (there is a pthread api for that). If you are still worried see my notes below about using robust mutexes anyway.

I just want to make it plain & clear that you are going to pay in performance if you want to use robust futexes in a multi-threaded application. The main use of robust futexes is to use them as synchronization primitives in multi-process applications where the chance of one component dying without killing the rest of the components is high compared to the same chance in a multi-threaded application where the abnormal death of a thread means the death of the entire application.

To use robust futexes in either a multi-threaded or a multi-process application you need to mark the futexes as robust by using the undocumented function pthread_mutexattr_setrobust(3). I've submitted a bug report to the manual pages maintainers to add documentation about that function. You need to pass PTHREAD_MUTEX_ROBUST to that function as opposed to PTHREAD_MUTEX_STALLED which is the default.

In a multi-threaded application marking the mutex as robust is all you have to do.

To use robust futexes in a multi-process application you need to also mark the futex as being shared across processes by calling the (fortunately documented) function pthread_mutexattr_setpshared(3) and pass PTHREAD_PROCESS_SHARED to it. This is opposed to the default PTHREAD_PROCESS_PRIVATE.

Actually in strace(1) you will not see acquisition and release of the locks but you will see calls to set_robust_list(2) if your futex is robust.

I hope this helps.