linux-kernel - futex breakage in 4.9 stable branch

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Date:   Mon, 01 Mar 2021 01:13:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        torvalds@...ux-foundation.org, stable@...r.kernel.org,
        Lee Jones <lee.jones@...aro.org>,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Cc:     lwn@....net, jslaby@...e.cz
Subject: futex breakage in 4.9 stable branch

On Tue, 2021-02-23 at 15:00 +0100, Greg Kroah-Hartman wrote:
> I'm announcing the release of the 4.9.258 kernel.
> 
> All users of the 4.9 kernel series must upgrade.
> 
> The updated 4.9.y git tree can be found at:
>         git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git linux-4.9.y
> and can be browsed at the normal kernel.org git web browser:
>         

The backported futex fixes are still incomplete/broken in this version.
If I enable lockdep and run the futex self-tests (from 5.10):

- on 4.9.246, they pass with no lockdep output
- on 4.9.257 and 4.9.258, they pass but futex_requeue_pi trigers a
  lockdep splat

I have a local branch that essentially updates futex and rtmutex in
4.9-stable to match 4.14-stable.  With this, the tests pass and lockdep
is happy.

Unfortunately, that branch has about another 60 commits.  Further, the
more we change futex in 4.9, the more difficult it is going to be to
update the 4.9-rt branch.  But I don't see any better option available
at the moment.

Thoughts?

Ben.

-- 
Ben Hutchings
I'm always amazed by the number of people who take up solipsism because
they heard someone else explain it. - E*Borg on alt.fan.pratchett

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