Re: [PATCH] printk: inject caller information into the body of message

From: Tetsuo Handa
Date: Mon Oct 08 2018 - 16:48:45 EST

On 2018/10/09 1:03, Petr Mladek wrote:
> On Mon 2018-10-08 19:31:58, Tetsuo Handa wrote:
>> A structure named "struct printk_buffer" is introduced for buffering
>> up to LOG_LINE_MAX bytes of printk() output which did not end with '\n'.
>> A caller is allowed to allocate/free "struct printk_buffer" using
>> kzalloc()/kfree() if that caller is in a location where it is possible
>> to do so.
>> A macro named "DEFINE_PRINTK_BUFFER()" is defined for allocating
>> "struct printk_buffer" from the stack memory or in the .bss section.
>> But since sizeof("struct printk_buffer") is nearly 1KB, it might not be
>> preferable to allocate "struct printk_buffer" from the stack memory.
>> In that case, a caller can use best-effort buffering mode. Two functions
>> get_printk_buffer() and put_printk_buffer() are provided for that mode.
>> get_printk_buffer() tries to assign a "struct printk_buffer" from
>> statically preallocated array. It returns NULL if all static
>> "struct printk_buffer" are in use.
>> put_printk_buffer() flushes and releases the "struct printk_buffer".
>> put_printk_buffer() must match corresponding get_printk_buffer() as with
>> rcu_read_unlock() must match corresponding rcu_read_lock().
> One problem with this API is when it is used in more complicated code
> and put_printk_buffer() is not called in some path. I mean leaking.
> We might get out of buffers easily.

Then, as an debugging config option for statically preallocated buffers,
we could record how get_printk_buffer() was called, like lockdep records
where a lock was taken.

> A solution might be to store some information about the owner and
> put the buffer also when a non-buffered printk is called from
> the same context.
> It might even make it easier to use. If we are able to guess the
> buffer by the context, we do not need to pass it as an argument.

It would be nice if we can omit passing "struct printk_buffer" argument.
But that results in "implicit contexts" which Linus has rejected
( ).

> Well, I would like to avoid having the buffer connected with CPU.
> It would require to disable preemption in get_printk_buffer().
> IMHO, it would be a unintuitive and even unwanted side effect.

get_printk_buffer() is connected with the context who called "struct printk_buffer".
There is no need to disable preemption.

> Best Regards,
> Petr
> PS: I am sorry for the late reply. I was busy with some other
> important stuff. I still have to think more about it and look
> mode deeply into the implementation.

No problem. Thank you for replying.

> In each case, we need to be careful about the design.
> The API has to be easy and safe to use. Also the implementation
> should not complicate the printk design too much.
> It looks promising. Also there is a high chance that it would
> be much more straightforward than the current code around
> the cont buffer ;-)

We could eventually remove "struct cont" buffer.