Re: [PATCH printk v3 14/15] printk: extend console_lock for proper kthread support

[John Ogness <john.ogness@linutronix.de>]

On 2022-04-22, Petr Mladek <pmladek@suse.com> wrote:
> Another problem is that the ordering is not stable. The console
> might come and go.

The console list is protected by @console_sem, so it wouldn't be an
actual problem. The real issue is that lockdep would not like it. A new
lockdep class would need to be setup for each register_console().

>> Anyway, I will first look into the nested locking solution. That
>> seems more promising to me and it would go a long way to simplify the
>> locking hierarchy.
>
> Please, do not spend too much time on this. The solution must be
> simple in principle. If it gets complicated than it will likely
> be worse than the current code.

Sure. The goal is to simplify. The only complexity will be doing in a
way that allow lockdep to understand it.

> Alternative solution would be to reduce the number of variables
> affected by the race. I mean:
>
>    + replace CON_THB_BLOCKED flag with con->blocked to avoid
>      the needed of READ_ONCE()/WRITE_ONCE().
>
>    + check con->blocked right after taking con->lock in
>      printk_kthread_func() so that all the other accesses are
>      safe.

Honestly, I would prefer this to what v4 is doing. The only reason
CON_THD_BLOCKED is a flag is to save space. But we are only talking
about a few bytes being saved. There aren't that many consoles.

It would be a very simple change. Literally just replacing the 3 lines
that set/clear CON_THD_BLOCKED and replacing/reordering the 2 lines that
check the flag. Then all the READ_ONCE/WRITE_ONCE to @flags could be
removed.

John