linux-kernel - Re: [PATCH v2] exit: Move preemption fixup up, move blocking operations down

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Date:   Thu, 5 Mar 2020 17:13:44 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] exit: Move preemption fixup up, move blocking
 operations down

On Thu,  5 Mar 2020 23:06:57 +0100
Jann Horn <jannh@...gle.com> wrote:

> With CONFIG_DEBUG_ATOMIC_SLEEP=y and CONFIG_CGROUPS=y, kernel oopses in
> non-preemptible context look untidy; after the main oops, the kernel prints
> a "sleeping function called from invalid context" report because
> exit_signals() -> cgroup_threadgroup_change_begin() -> percpu_down_read()
> can sleep, and that happens before the preempt_count_set(PREEMPT_ENABLED)
> fixup.
> 
> It looks like the same thing applies to profile_task_exit() and
> kcov_task_exit().
> 
> Fix it by moving the preemption fixup up and the calls to
> profile_task_exit() and kcov_task_exit() down.
> 
> Fixes: 1dc0fffc48af ("sched/core: Robustify preemption leak checks")
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---



> @@ -732,6 +736,16 @@ void __noreturn do_exit(long code)
>  	 */
>  	set_fs(USER_DS);
>  
> +	if (unlikely(in_atomic())) {
> +		pr_info("note: %s[%d] exited with preempt_count %d\n",
> +			current->comm, task_pid_nr(current),
> +			preempt_count());

This should be more than a pr_info. It should also probably state the
"Dazed and confused, best to reboot" message.

Because if something crashed in a non preempt section, it may likely be
holding a lock that it will never release, causing a soon to be deadlock!

-- Steve


> +		preempt_count_set(PREEMPT_ENABLED);
> +	}
> +
> +	profile_task_exit(tsk);
> +	kcov_task_exit(tsk);
> +
>  	ptrace_event(PTRACE_EVENT_EXIT, code);
>  
>  	validate_creds_for_do_exit(tsk);
> @@ -749,13 +763,6 @@ void __noreturn do_exit(long code)
>  
>  	exit_signals(tsk);  /* sets PF_EXITING */
>  
> -	if (unlikely(in_atomic())) {
> -		pr_info("note: %s[%d] exited with preempt_count %d\n",
> -			current->comm, task_pid_nr(current),
> -			preempt_count());
> -		preempt_count_set(PREEMPT_ENABLED);
> -	}
> -
>  	/* sync mm's RSS info before statistics gathering */
>  	if (tsk->mm)
>  		sync_mm_rss(tsk->mm);
> 
> base-commit: 9f65ed5fe41ce08ed1cb1f6a950f9ec694c142ad