Date:   Mon, 20 Mar 2023 16:04:07 -0400
From:   Joe Korty <joe.korty@concurrent-rt.com>
To:     "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH 5.10.162-rt78] Restore initialization of
 wake_q_sleeper.next in fork.c
Message-ID: <20230320200407.GA39185@zipoli.concurrent-rt.com>
Reply-To: Joe Korty <joe.korty@concurrent-rt.com>
References: <20230320193731.GA36840@zipoli.concurrent-rt.com>
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On Mon, Mar 20, 2023 at 05:00:13PM -0300, Luis Claudio R. Goncalves wrote:
> On Mon, Mar 20, 2023 at 03:37:31PM -0400, Joe Korty wrote:
> > In the transition from 5.10.158-rt77 to 5.10.162-rt78,
> > the initialization of task_struct::wake_q_sleeper.next
> > was dropped.  Restore it.
> > 
> > This appears to be only a problem in 5.10.  5.15 does not
> > have wake_q_sleeper; 4.19 does have it but its initialization
> > there is still present.
> > 
> > The 5.10.162-rt78 patch that damaged fork.c is:
> > 
> >    0170-locking-rtmutex-add-sleeping-lock-implementation.patch
> > 
> > I do not have a simple test that brings out this problem.
> > My test consists of a shell script and eight binaries,
> > all of which were written in Ada.  strace shows that it
> > does a few thousand forks in rapid succession.  One of the
> > forks stalls out, after which no fork after that returns.
> > Eventually the 122 second stallout occurs and a large
> > number of threads are shown to be waiting for tasklist
> > lock, either in do_exit or in copy_process.  The kernel
> > .config has rt and many debug features enabled, lockdep
> > included.
> 
> Joe, thank you for investigating that problem and for writing a patch.
> 
> Earlier today Steffen Dirkwinkel sent a similar patch:
> 
>     https://lore.kernel.org/all/20230320080347.32434-1-linux@steffen.cc/
> 
> Would you mind giving your ACK to his patch? I have that patch queued for
> my next build already.

Acked-by: Joe Korty <joe.korty@concurrent-rt.com>