linux-kernel - Re: futex breakage in 4.9 stable branch

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Message-ID: <YD0kkNH+I4xyoTwy@decadent.org.uk>
Date:   Mon, 1 Mar 2021 18:29:52 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     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>, lwn@....net,
        jslaby@...e.cz
Subject: Re: futex breakage in 4.9 stable branch

On Mon, Mar 01, 2021 at 09:07:03AM +0100, Greg Kroah-Hartman wrote:
> On Mon, Mar 01, 2021 at 01:13:08AM +0100, Ben Hutchings wrote:
> > 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.

I have now rebased that on top of 4.9.258, and there are "only" 39
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?
> 
> There were some posted futex fixes for 4.9 (and 4.4) on the stable list
> that I have not gotten to yet.
> 
> Hopefully after these are merged (this week), these issues will be
> resolved.

I'm afraid they are not sufficient.

> If not, then yes, they need to be fixed and any help you can provide
> would be appreciated.
> 
> As for "difficulty", yes, it's rough, but the changes backported were
> required, for obvious reasons :(

I had another look at the locking bug and I was able to make a series
of 7 commits (on top of the 2 already queued) that is sufficient to
make lockdep happy.  But I am not very confident that there won't be
other regressions.  I'll send that over shortly.

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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