Messages in this thread | | | From | John Ogness <> | Subject | Re: [patch RFC 06/29] printk: Protect [un]register_console() with a mutex | Date | Mon, 19 Sep 2022 14:55:50 +0206 |
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On 2022-09-14, Sergey Senozhatsky <senozhatsky@chromium.org> wrote: > On (22/09/11 00:27), Thomas Gleixner wrote: > [..] >> + * 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); > [..] >> /* >> * Helper macros to handle lockdep when locking/unlocking console_sem. We use >> * macros instead of functions so that _RET_IP_ contains useful information. >> @@ -2978,17 +3008,21 @@ struct tty_driver *console_device(int *i >> void console_stop(struct console *console) >> { >> __pr_flush(console, 1000, true); >> + console_list_lock(); >> console_lock(); >> console->flags &= ~CON_ENABLED; >> console_unlock(); >> + console_list_unlock(); >> } >> EXPORT_SYMBOL(console_stop); >> >> void console_start(struct console *console) >> { >> + console_list_lock(); >> console_lock(); >> console->flags |= CON_ENABLED; >> console_unlock(); >> + console_list_unlock(); >> __pr_flush(console, 1000, true); >> } >> EXPORT_SYMBOL(console_start); > > So the comment says that list lock (console_mutex) is to serialize > register/unregister, but then we take it in stop/start as well. What > does list lock protect us against in start/stop? console->flags reader > (console_is_usable()) does not take list lock, it's called under console > lock and console->flags writers (console_unregister() and console_stop()) > modify console->flags under console_lock.
Currently all writers to console->flags are holding the console_lock. However, there are console->flags readers that do _not_ hold the console_lock (register_console, unregister_console, printk_late_init).
Aside from adding list synchronization, the list lock also provides the missing console->flags synchronization. Now all console->flags writers hold the list lock _and_ console_lock. A console->flags reader can hold either the list lock or the console_lock.
Since console_start and console_stop are console->flags writers, they also need to take the list lock. I agree that this should be mentioned in the commit message and code comments.
The follow-up patch in the series only deals with list/flags readers. Therefore I think the change to console_stop/console_start belongs in this patch, which focusses on fixing synchronization.
John
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