Re: [patch 0/6] futex: Bugfixes and FUTEX_LOCK_PI2

[Peter Zijlstra]

On Thu, Apr 22, 2021 at 09:44:17PM +0200, Thomas Gleixner wrote:
> The following series started off looking into supporting selectable clocks
> for FUTEX_LOCK_PI which is hardcoded to CLOCK_REALTIME and cannot be
> changed.
> 
> On the way I found two bugs related to the timeout handling:
> 
>   - The allowance for FUTEX_WAIT to use FUTEX_CLOCK_REALTIME is broken and
>     never worked.
> 
>   - The recent time namespace support wreckaged FUTEX_LOCK_PI timeouts when
>     the task belongs to a namespace which has an CLOCK_MONOTONIC offset.
> 
> Both should have been caught by that Gleixner dude when merging them,
> but it seems he's getting old.
> 
> Not having a selectable clock for PI futexes is inconsistent because all
> other interfaces have it. Unfortunately this was figured out by glibc folks
> quite some time ago, but nobody told us :(
> 
> The nasty hack to support it would be to treat FUTEX_CLOCK_REALTIME inverse
> for FUTEX_LOCK_PI, but that's a horrible idea. Adding a new flag to the
> futex op, i.e. FUTEX_CLOCK_MONOTONIC would be possible, but that's yet
> another variant which makes is harder for libraries to have a consistent
> clock selection handling.
> 
> So I went the way to let FUTEX_LOCK_PI alone and to add FUTEX_LOCK_PI2
> which handles the clocks the same way as the other operands.
> 
> Thoughts?

With the missing FUTEX_LOCK_PI2 in #6, as spotted by André Almeida, fixed:

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

It's all somewhat sad, but I don't see any other way out of this. Using
LOCK_PI2 will be a fairly horrible pile of hacks on the userspace side
of things given they need to first detect it's presence etc., but that
seems unavoidable whatever we do :/