Re: [musl] robust list not issuing FUTEX_WAKE

[Rich Felker <dalias@libc.org>]

On Wed, Apr 20, 2022 at 07:56:17PM -0400, Rich Felker wrote:
> 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).

Also: you only get the FUTEX_WAIT if the waiters but (bit 31) was set
when the owner died.