Re: [RFC PATCH 0/1] Add FUTEX_SPIN operation

[Christian Brauner <brauner@kernel.org>]

On Thu, May 02, 2024 at 11:51:56AM +0200, Florian Weimer wrote:
> * Christian Brauner:
> 
> > Unless I'm missing something the question here is PID (as in TGID aka
> > thread-group leader id gotten via getpid()) vs TID (thread specific id
> > gotten via gettid()). You want the thread-specific id as you want to
> > interact with the futex state of a specific thread not the thread-group
> > leader.
> >
> > Aside from that TIDs are subject to the same race conditions that PIDs
> > are. They are allocated from the same pool (see alloc_pid()).
> 
> For most mutex types (but not robust mutexes), it is undefined in
> userspace if a thread exits while it has locked a mutex.  Such a usage
> condition would ensure that the race doesn't happen, I believe.

The argument is a bit shaky imho because the race not being able to
happen is predicated on no one being careless enough to exit with a
mutex held. That doesn't do anything against someone doing it on
purpose.

> 
> From a glibc perspective, we typically cannot use long-term file
> descriptors (that are kept open across function calls) because some
> applications do not expect them, or even close them behind our back.

Yeah, good point. Note, I suggested it as an extension not as a
replacement for the TID. I still think it would be a useful extension in
general.