Subject: Re: [PATCH next v1 2/3] printk: remove safe buffers

From: John Ogness <[email-masked]>
Date: Fri, 26 Mar 2021 12:12:37 +0100

On 2021-03-23, Petr Mladek <[email-masked]> wrote:
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -1142,8 +1126,6 @@ void __init setup_log_buf(int early)
>>  		 new_descs, ilog2(new_descs_count),
>>  		 new_infos);
>>  
>> -	printk_safe_enter_irqsave(flags);
>> -
>>  	log_buf_len = new_log_buf_len;
>>  	log_buf = new_log_buf;
>>  	new_log_buf_len = 0;
>> @@ -1159,8 +1141,6 @@ void __init setup_log_buf(int early)
>>  	 */
>>  	prb = &printk_rb_dynamic;
>>  
>> -	printk_safe_exit_irqrestore(flags);
>
> This will allow to add new messages from the IRQ context when we
> are copying them to the new buffer. They might get lost in
> the small race window.
>
> Also the messages from NMI might get lost because they are not
> longer stored in the per-CPU buffer.
>
> A possible solution might be to do something like this:
>
> 	prb_for_each_record(0, &printk_rb_static, seq, &r)
> 		free -= add_to_rb(&printk_rb_dynamic, &r);
>
> 	prb = &printk_rb_dynamic;
>
> 	/*
> 	 * Copy the remaining messages that might have appeared
> 	 * from IRQ or NMI context after we ended copying and
> 	 * before we switched the buffers. They must be finalized
> 	 * because only one CPU is up at this stage.
> 	 */
> 	prb_for_each_record(seq, &printk_rb_static, seq, &r)
> 		free -= add_to_rb(&printk_rb_dynamic, &r);

OK. I'll probably rework it some and combine it with the "dropped" test
so that we can identify if messages were dropped during the transition
(because of static ringbuffer overrun).

>> -
>>  	if (seq != prb_next_seq(&printk_rb_static)) {
>>  		pr_err("dropped %llu messages\n",
>>  		       prb_next_seq(&printk_rb_static) - seq);
>> @@ -2666,7 +2631,6 @@ void console_unlock(void)
>>  		size_t ext_len = 0;
>>  		size_t len;
>>  
>> -		printk_safe_enter_irqsave(flags);
>>  skip:
>>  		if (!prb_read_valid(prb, console_seq, &r))
>>  			break;
>> @@ -2711,6 +2675,8 @@ void console_unlock(void)
>>  				printk_time);
>>  		console_seq++;
>>  
>> +		printk_safe_enter_irqsave(flags);
>
> What is the purpose of the printk_safe context here, please?

console_lock_spinning_enable() needs to be called with interrupts
disabled. I should have just used local_irq_save().

I could add local_irq_save() to console_lock_spinning_enable() and
restore them at the end of console_lock_spinning_disable_and_check(),
but then I would need to add a @flags argument to both functions. I
think it is simpler to just do the disable/enable from the caller,
console_unlock().

BTW, I could not find any sane way of disabling interrupts via a
raw_spin_lock_irqsave() of @console_owner_lock because of the how it is
used with lockdep. In particular for
console_lock_spinning_disable_and_check().

John Ogness