On 2021-03-23, Petr Mladek <pmla...@suse.com> 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
