Re: [PATCH v2 3/4] printk: Avoid livelock with heavy printk during panic

[Petr Mladek <pmladek@suse.com>]

On Wed 2022-01-26 15:02:35, Stephen Brennan wrote:
> During panic(), if another CPU is writing heavily the kernel log (e.g.
> via /dev/kmsg), then the panic CPU may livelock writing out its messages
> to the console. Note when too many messages are dropped during panic and
> suppress further printk, except from the panic CPU. This could result in
> some important messages being dropped. However, messages are already
> being dropped, so this approach at least prevents a livelock.
> 
> Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
> ---
> 
> Notes:
>     v2: Add pr_warn when we suppress printk on non-panic CPU
> 
>  kernel/printk/printk.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 20b4b71a1a07..18107db118d4 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -93,6 +93,12 @@ EXPORT_SYMBOL_GPL(console_drivers);
>   */
>  int __read_mostly suppress_printk;
>  
> +/*
> + * During panic, heavy printk by other CPUs can delay the
> + * panic and risk deadlock on console resources.
> + */
> +int __read_mostly suppress_panic_printk;
> +
>  #ifdef CONFIG_LOCKDEP
>  static struct lockdep_map console_lock_dep_map = {
>  	.name = "console_lock"
> @@ -2228,6 +2234,10 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	if (unlikely(suppress_printk))
>  		return 0;
>  
> +	if (unlikely(suppress_panic_printk) &&
> +	    atomic_read(&panic_cpu) != raw_smp_processor_id())
> +		return 0;
> +
>  	if (level == LOGLEVEL_SCHED) {
>  		level = LOGLEVEL_DEFAULT;
>  		in_sched = true;
> @@ -2613,6 +2623,7 @@ void console_unlock(void)
>  {
>  	static char ext_text[CONSOLE_EXT_LOG_MAX];
>  	static char text[CONSOLE_LOG_MAX];
> +	static int panic_console_dropped;
>  	unsigned long flags;
>  	bool do_cond_resched, retry;
>  	struct printk_info info;
> @@ -2667,6 +2678,11 @@ void console_unlock(void)
>  		if (console_seq != r.info->seq) {
>  			console_dropped += r.info->seq - console_seq;
>  			console_seq = r.info->seq;
> +			if (panic_in_progress() && panic_console_dropped++ > 10) {
> +				suppress_panic_printk = 1;
> +				pr_warn("Too many dropped messages. "
> +				        "Supress messages on non-panic CPUs to prevent livelock.\n");

It looks like the message might be printed more times when
panic_console_dropped++ > 10.

In theory, no message can be lost after we disable printk on another
CPUs. But the code might evolve in the future. Let's make it
more error-proof.

We could use (panic_console_dropped++ == 10) or pr_warn_once() or
both.

I prefer using pr_warn_once() because it looks the most error-proof.


Nit: printk() has exceptions from the 80 chars/line rule.
     The message string should be on a single line. It helps
     to find it using "git grep". I think that even checkpatch.pl
     handles this correctly.


With pr_warn_once() and message in single line:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr


PS: I could fix the two problems when pushing to git. But there
    is still time to send v3. I have vacation the following week
    with limited internet access. I am not going to rush it into
    linux before I leave, ...