register_console: was: Re: [PATCH printk v2 11/12] printk: extend console_lock for proper kthread support

[Petr Mladek <pmladek@suse.com>]

On Tue 2022-04-05 15:31:34, John Ogness wrote:
> Currently threaded console printers synchronize against each
> other using console_lock(). However, different console drivers
> are unrelated and do not require any synchronization between
> each other. Removing the synchronization between the threaded
> console printers will allow each console to print at its own
> speed.
> 
> But the threaded consoles printers do still need to synchronize
> against console_lock() callers. Introduce a per-console mutex
> and a new console flag CON_THD_BLOCKED to provide this
> synchronization.
> 
> console_lock() is modified so that it must acquire the mutex
> of each console in order to set the CON_THD_BLOCKED flag.
> Console printing threads will acquire their mutex while
> printing a record. If CON_THD_BLOCKED was set, the thread will
> go back to sleep instead of printing.
> 
> The reason for the CON_THD_BLOCKED flag is so that
> console_lock() callers do not need to acquire multiple console
> mutexes simultaneously, which would introduce unnecessary
> complexity due to nested mutex locking.
> 
> However, threaded console printers also need to synchronize
> against console_trylock() callers. Since console_trylock() may
> be called from any context, the per-console mutex cannot be
> used for this synchronization. (mutex_trylock() cannot be
> called from atomic contexts.) Introduce a global atomic counter
> to identify if any threaded printers are active. The threaded
> printers will also check the atomic counter to identify if the
> console has been locked by another task via console_trylock().
> 
> Note that @console_sem is still used to provide synchronization
> between console_lock() and console_trylock() callers.
> 
> A locking overview for console_lock(), console_trylock(), and the
> threaded printers is as follows (pseudo code):
> 
> console_lock()
> {
>         down(&console_sem);
>         for_each_console(con) {
>                 mutex_lock(&con->lock);
>                 con->flags |= CON_THD_BLOCKED;
>                 mutex_unlock(&con->lock);
>         }
>         /* console lock acquired */
> }
> 
> console_trylock()
> {
>         if (down_trylock(&console_sem) == 0) {
>                 if (atomic_cmpxchg(&console_kthreads_active, 0, -1) == 0) {
>                         /* console lock acquired */
>                 }
>         }
> }
> 
> threaded_printer()
> {
>         mutex_lock(&con->lock);
>         if (!(con->flags & CON_THD_BLOCKED)) {
> 		/* console_lock() callers blocked */
> 
>                 if (atomic_inc_unless_negative(&console_kthreads_active)) {
>                         /* console_trylock() callers blocked */
> 
>                         con->write();
> 
>                         atomic_dec(&console_lock_count);
>                 }
>         }
>         mutex_unlock(&con->lock);
> }
> 
> The console owner and waiter logic now only applies between contexts
> that have taken the console lock. Since threaded printers never take
> the console lock, they do not have a console lock to handover to
> other contexts.
> 
> @panic_console_dropped must change to atomic_t since it is no longer
> protected exclusively by the console lock.
> 
> Since threaded printers remain asleep if they see that the console
> is locked, they now must be explicitly woken in __console_unlock().
> This means wake_up_klogd() calls following a console_unlock() are
> no longer necessary and are removed.

BTW: I really like the commit message.

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3230,7 +3335,11 @@ void register_console(struct console *newcon)
>  	if (newcon->flags & CON_EXTENDED)
>  		nr_ext_console_drivers++;
>  
> +	if (console_kthreads_blocked)

I think that this is actully always true because it is called under
console_lock().

> +		newcon->flags |= CON_THD_BLOCKED;

> +
>  	newcon->dropped = 0;
> +	mutex_init(&newcon->lock);
>  	if (newcon->flags & CON_PRINTBUFFER) {
>  		/* Get a consistent copy of @syslog_seq. */
>  		mutex_lock(&syslog_lock);

Best Regards,
Petr