Re: [musl] robust list not issuing FUTEX_WAKE

[Rich Felker <dalias@libc.org>]

On Wed, Apr 20, 2022 at 04:46:23PM -0700, Leonid Shamis wrote:
> Hey Musl Folks,
> 
> I'm trying to understand the futex robust list and it's not quite working
> how I would expect it to from a reading of the man pages.
> 
> In a minimal example, I'm getting the futex changed to FUTEX_OWNER_DIED
> instead of FUTEX_OWNER_DIED|tid and I'm not getting a FUTEX_WAKE event.
> Any idea why this might be?
> 
> Happy to share the minimal example.

Can you clarify if you're trying to use robust mutexes under musl or
roll your own thing using the kernel robust list directly? I don't
think I'm understanding what you're confused about. If the owner of a
robust mutex dies, its tid can no longer be in the futex word because
that tid is immediately available to be assigned to a new task, and
the next thread trying to lock the mutex would not be able to
distinguish whether it's owned by a new thread with that tid, or
available.

So as far as the kernel is concerned, FUTEX_OWNER_DIED is more of a
value than a flag. It's what the futex word gets set to when the owner
dies with it held. In musl, we use FUTEX_OWNER_DIED as a flag as well
for a robust mutex whose old owner died and who has a new owner but
hasn't yet called pthread_mutex_consistent (in which case it would
become unrecoverable on unlock).

Rich