Re: [PATCH rcu 1/2] torture: Optionally flush printk() buffers before powering off

[John Ogness <john.ogness@linutronix.de>]

Added CC the rest of the printk folks since this would be the first use
of pr_flush() outside of the printk subsystem. And it is a controversial
usage, since it is used to work around potential (future) bugs in
printk.

On 2022-08-31, "Paul E. McKenney" <paulmck@kernel.org> wrote:
> The rcutorture test suite produces quite a bit of console output at
> the end of a test.  This means that the new-in-2022 printk() kthreads
> are likely to be in the process of flushing output at the time of the
> torture_shutdown() function's call to kernel_power_off().  Normally,
> rcutorture relies on printk() to flush any pending output upon shutdown,
> the better to detect bugs in this area, for example, the one introduced
> by 8e274732115f ("printk: extend console_lock for per-console locking").
> However, once such a bug is detected and reported, it is necessary to
> test the rest of the system, without noise from the already-reported bug.

Since the printk kthread implementation has been reverted, this is no
longer an issue. When a kthread implementation is re-introduced, this
issue will need to have been considered and handled.

IMHO this patch is premature at this time and hopefully will never
become necessary (in the form of a bug workaround).

> This commit therefore adds a torture.printk_shutdown_bug_workaround
> kernel parameter, which causes torture_shutdown() to invoke pr_flush(),
> and print an informative message on the console, immediately before
> invoking kernel_power_off().  When this kernel parameter is not specified,
> it is up to printk() to flush its own buffers.
>
> Suggested-by: John Ogness <john.ogness@linutronix.de>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
>  kernel/torture.c                                | 7 +++++++
>  2 files changed, 13 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index d7f30902fda02..ba1b8776aab83 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6143,6 +6143,12 @@
>  			are running concurrently, especially on systems
>  			with rotating-rust storage.
>  
> +	torture.printk_shutdown_bug_workaround= [KNL]
> +			Execute pr_flush(1000, true) just before invoking
> +			kernel_power_off() to work around any bugs that
> +			might prevent printk() from flushing its buffers
> +			at shutdown time.
> +
>  	torture.verbose_sleep_frequency= [KNL]
>  			Specifies how many verbose printk()s should be
>  			emitted between each sleep.  The default of zero
> diff --git a/kernel/torture.c b/kernel/torture.c
> index 789aeb0e1159c..7cd2016b02076 100644
> --- a/kernel/torture.c
> +++ b/kernel/torture.c
> @@ -48,6 +48,9 @@ module_param(disable_onoff_at_boot, bool, 0444);
>  static bool ftrace_dump_at_shutdown;
>  module_param(ftrace_dump_at_shutdown, bool, 0444);
>  
> +static bool printk_shutdown_bug_workaround;
> +module_param(printk_shutdown_bug_workaround, bool, 0444);
> +
>  static int verbose_sleep_frequency;
>  module_param(verbose_sleep_frequency, int, 0444);
>  
> @@ -651,6 +654,10 @@ static int torture_shutdown(void *arg)
>  		VERBOSE_TOROUT_STRING("No torture_shutdown_hook(), skipping.");
>  	if (ftrace_dump_at_shutdown)
>  		rcu_ftrace_dump(DUMP_ALL);
> +	if (printk_shutdown_bug_workaround) {
> +		pr_info("%s: Flushing printk() buffers at power-down time.\n", __func__);
> +		pr_flush(1000, true);
> +	}
>  	kernel_power_off();	/* Shut down the system. */
>  	return 0;
>  }
> -- 
> 2.31.1.189.g2e36527f23

John Ogness