Messages in this thread | | | Date | Tue, 27 Sep 2022 11:56:27 +0200 | From | Petr Mladek <> | Subject | Re: [patch RFC 06/29] printk: Protect [un]register_console() with a mutex |
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On Sun 2022-09-11 00:27:41, Thomas Gleixner wrote: > Unprotected list walks are a brilliant idea. Especially in the context of > hotpluggable consoles.
Yeah, it is crazy. And it is there probably since the beginning.
> --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -79,10 +79,14 @@ int oops_in_progress; > EXPORT_SYMBOL(oops_in_progress); > > /* > - * console_sem protects the console_drivers list, and also > - * provides serialisation for access to the entire console > - * driver system. > + * console_sem protects the console_drivers list, and also provides > + * serialization for access to the entire console driver system. > + * > + * console_mutex serializes register/unregister. console_sem has to be > + * taken for any list manipulation inside the console_mutex locked > + * section to keep the console BKL machinery happy. > */ > +static DEFINE_MUTEX(console_mutex); > static DEFINE_SEMAPHORE(console_sem); > struct console *console_drivers; > EXPORT_SYMBOL_GPL(console_drivers); > @@ -220,6 +230,26 @@ int devkmsg_sysctl_set_loglvl(struct ctl > } > #endif /* CONFIG_PRINTK && CONFIG_SYSCTL */ > > +/** > + * console_list_lock - Lock the console list > + * > + * For non-console related list walks, e.g. procfs, sysfs... > + */ > +void console_list_lock(void) > +{ > + mutex_lock(&console_mutex); > +} > + > +/** > + * console_list_unlock - Unlock the console list > + * > + * Counterpart to console_list_lock() > + */ > +void console_list_unlock(void) > +{ > + mutex_unlock(&console_mutex); > +} > + > /* > * Helper macros to handle lockdep when locking/unlocking console_sem. We use > * macros instead of functions so that _RET_IP_ contains useful information. > @@ -3107,13 +3143,14 @@ void register_console(struct console *ne > 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.
The ideal solution would be take console_lock() here. We (me and Sergey) never did it because con->match() and con->setup() callbacks were called in try_enable_*console(). We were afraid that some might want to take console_lock() and it could create a deadlock. There were too many drivers and we did not found time to check them all. And it had low priority because nobody reported problems.
A good enough solution might be call this under the later added srcu_read_lock(&console_srcu) and use for_each_console_srcu().
The srcu walk would prevent seeing broken list. Obviously, the code might see outdated list and do bad decisions:
+ try to enable the same console twice
+ enable more consoles by default in try_enable_default_console()
+ associate more consoles with /dev/console, see CON_CONSDEV in try_enable_preferred_console() and try_enable_default_console()
If we race then we could end up with more consoles enabled by default and with more consoles with CON_CONSDEV flag.
IMHO, the rcu walk is an acceptable and conservative solution. Registering the same driver twice is hard to imagine at all. And I have never seen reports about too many default consoles or CON_CONSDEV flags.
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?
> + for_each_registered_console(con) { > if (WARN(con == newcon, "console '%s%d' already registered\n", > con->name, con->index)) > - return; > + goto unlock; > } > > - for_each_console(con) { > + for_each_registered_console(con) { > if (con->flags & CON_BOOT) > bootcon_enabled = true; > else
Best Regards, Petr
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