Messages in this thread | | | Date | Wed, 28 Sep 2022 18:46:42 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH printk 06/18] printk: Protect [un]register_console() with a mutex |
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On (22/09/27 17:16), Petr Mladek wrote: [..] > > +static int console_unregister_locked(struct console *console); > > + > > /* > > * The console driver calls this routine during kernel initialization > > * to register the console printing procedure with printk() and to > > @@ -3107,13 +3148,14 @@ void register_console(struct console *newcon) > > bool realcon_enabled = false; > > int err; > > > > - for_each_console(con) { > > + console_list_lock(); > > Hmm, the new mutex is really nasty. It has very strange semantic. > It makes the locking even more complicated.
[..]
I fully agree with everything you said. This lock nesting made me scratch my head wondering was it previous CPU hotplug code that had multiple nested locks or was it something else?
> Anyway, I would like to avoid adding console_mutex. From my POV, > it is a hack that complicates the code. Taking console_lock() > should be enough. Using rcu walk would be good enough. > > Do I miss something, please? > > Or is this part of some strategy to remove console_sem later, please?
So I can only explain what potential I saw in list lock: the idea that third party that iterates over consoles lists does not stop entire console output machinery, and, moreover, that third party does not flush pending messages once it's done with the business it had to do under console_sem. E.g. it can be a systemd or any other user-space process doing something with /dev/tty, which can suddenly stop all consoles output (console_lock()) and then also has to flush pending kernel messages (console_unlock()). Was this goal, however, fully achieved - no, a third party that wants to ->flags &= ~CON_ENABLED a particular console still stops the entire console output (and flushes pending messages, unless handover-ed).
I like what you suggested with srcu.
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