Re: [PATCH printk v3 5/6] printk: introduce console_get_next_message() and console_message

[Petr Mladek]

On Wed 2023-01-04 11:32:30, John Ogness wrote:
> On 2023-01-03, Petr Mladek <pmladek@xxxxxxxx> wrote:
> >> The current atomic console proposal allocates 1x cbuf per-cpu and 4x
> >> meta-data per-cpu. Different contexts of a cpu will have different
> >> meta-data, but all the contexts of a cpu will share the same cbuf.
> >>
> >> If cbufs become embedded in cmsg, then we would allocate 1x cmsg
> >> per-cpu. But the atomic consoles would still need their own 4x per-cpu
> >> meta-data.
> >
> > Do we really need 4x the meta data?
> 
> Having per-context meta-data helps minimize data clobbering. For
> example, the message-formatting functions would not need to worry about
> @cmsg->len changing underneath them. This can be solved with a
> READ_ONCE() to a local variable and the function only using the local
> copy, but it will mean more copying of variables.

I see. I agree that a separate structure would avoid these problems.

> > The metadata describe the state of the buffer. Using the buffer in one
> > context invalidates the metadata in the other context.
> 
> Yes, but the message-formatting functions are the ones preparing that
> meta-data. They must then be able to handle an interrupting context
> changing that meta-data.
> 
> >> For v4 I will drop the console_buffers struct. I will use your
> >> suggestion.
> >
> > Please, do not do it just to make me happy. My intention
> > is to keep things simple. And keeping the two structures
> > synced needs an extra code.
> >
> > If you are sure that the separation will really be needed
> > in the future then feel free to keep the two structures.
> 
> Currently the message-formatting functions do not care about clobbering
> of the text buffers. They blindly just move things around using the
> meta-data as safety boundaries. This can lead to a formatted-buffer that
> contains garbage, but an interrupted context will not print that buffer
> in the end. The important thing is that the garbage was created safely.

Right.

> Avoiding a separate console_buffers structure may simplify the
> structures, but it requires robustifying the message-formatting
> functions to be tolerant for meta-data clobbering.
> 
> I am currently implementing such changes to see if it is feasible.

Please, do not spend too much time on this.

I think that the two structures make sense in the end. Just please
mention the metadata clobbering as the motivation in the commit
message.

Best Regards,
Petr