Date:   Mon, 20 Mar 2023 17:00:13 -0300
From:   "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
To:     Joe Korty <joe.korty@concurrent-rt.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: <ZBi7TbNgBAZL6tdw@uudg.org>
References: <20230320193731.GA36840@zipoli.concurrent-rt.com>
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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.

Thank you,
Luis
 
> Signed-off-by: Joe Korty <joe.korty@concurrent-rt.com
> 
> Index: b/kernel/fork.c
> ===================================================================
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -960,6 +960,7 @@ static struct task_struct *dup_task_stru
>  	tsk->splice_pipe = NULL;
>  	tsk->task_frag.page = NULL;
>  	tsk->wake_q.next = NULL;
> +	tsk->wake_q_sleeper.next = NULL;
>  	tsk->pf_io_worker = NULL;
>  
>  	account_kernel_stack(tsk, 1);
> 
---end quoted text---