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

[Stephen Brennan <stephen.s.brennan@oracle.com>]

On 1/27/22 06:50, Petr Mladek wrote:
> 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, ...
>      

Thank you Petr, I will go ahead and resolve things in this patch, and 
the others in a v3 soon. Regardless, no need to rush, please enjoy your 
vacation!

Thanks for all the review and guidance!
Stephen