Re: [musl] pthread_mutex_t shared between processes with different pid namespaces

[Rich Felker <dalias@libc.org>]

On Tue, Jan 28, 2025 at 02:22:31PM +0100, Daniele Personal wrote:
> Hello everyone,
> I'm working on a library linked by some processes in order to exchange
> information. Such library uses some pthread_mutex_t instances to safely
> read/write the information to exchange: the mutexes are created with
> the PTHREAD_PROCESS_SHARED and PTHREAD_MUTEX_ROBUST attributes and
> shared through shared memory mmapped by the processes.
> 
> Now, for certain reasons, I have to run one of the processes in a
> container and I found that, after a random interval of time, the
> process in the container got stuck in a pthread_mutex_lock without any
> reason.
> 
> After some investigation I figured out that if the container is started
> without pid namespace isolation everithing works like a charm.
> 
> So the questions: is the pid namespace isolation a problem when working
> with shared mutexes or should I investigate in other directions?
> If the problem is pid namespace isolation, what could be done to make
> it working apart from sharing the same pid namespace?
> 
> The actual development is based on musl 1.2.4 built with Yocto
> Scarthgap for aarch64 and arm.

Yes, the pid namespace boundary is your problem. Process-shared
mutexes only work on the same logical system with a unique set of
thread identifiers. If you're trying to share them across different
pid namespaces, the same pid/tid may refer to different
processes/threads in different ones, and it's not usable as a mutex
ownership identity.

If you want robust-mutex-like functionality that bridges pid
namespaces, sysv semaphores are probably your only option.

Rich