Re: [PATCH printk v5 1/1] printk: extend console_lock for per-console locking

[Petr Mladek]

On Mon 2022-04-25 23:04:28, 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 boolean field @blocked to provide this
> synchronization.
> 
> console_lock() is modified so that it must acquire the mutex
> of each console in order to set the @blocked field. Console
> printing threads will acquire their mutex while printing a
> record. If @blocked was set, the thread will go back to sleep
> instead of printing.
> 
> The reason for the @blocked boolean field 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. Also, a new field
> was chosen instead of adding a new @flags value so that the
> blocked status could be checked without concern of reading
> inconsistent values due to @flags updates from other contexts.
> 
> 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->blocked = true;
>                 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->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 via console_trylock(). Threaded
> printers never take the console_lock, so they do not have a
> console_lock to handover. Tasks that have used console_lock() will
> block the threaded printers using a mutex and if the console_lock
> is handed over to an atomic context, it would be unable to unblock
> the threaded printers. However, the console_trylock() case is
> really the only scenario that is interesting for handovers anyway.
> 
> @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.
> 
> Also note that threaded printers no longer need to check
> @console_suspended. The check for the @blocked field implicitly
> covers the suspended console case.
> 
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>

Nice, it it better than v4. I am going to push this for linux-next.

Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>

See below a comment about the possible future direction.

> ---
> 
>  Changes since v4 of this patch:
> 
>  - Use new @blocked field instead of CON_THD_BLOCKED flag.
> 
>  - Remove console_flags_set()/console_flags_clear() macros for
>    updating @flags (and remove their race comments).
> 
>  - For printer_should_wake() and printk_kthread_func(), check
>    @blocked before checking @flags.
> 
>  - Update commit message and comments appropriately.

Excellent work!

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3665,15 +3802,27 @@ static int printk_kthread_func(void *data)
>  		if (error)
>  			continue;
>  
> -		console_lock();
> +		error = mutex_lock_interruptible(&con->lock);
> +		if (error)
> +			continue;
>  
> -		if (console_suspended) {
> -			up_console_sem();
> +		if (con->blocked ||
> +		    !console_kthread_printing_tryenter()) {

It is great that you moved both conditions. I have just realized how
much information and functionality is accumulated here:

    + "con->blocked" is set when anyone else took @console_sem via
      console_lock() or when the console is suspended.

    + console_kthread_printing_tryenter() has two functions. It fails
      when anyone else took @console_sem via console_trylock().
      Also it blocks console_trylock(). Note that console_lock() is
      blocked because it has to wait for con->lock.

I missed the trylock part when proposed the more safe API in the other
thread, see https://lore.kernel.org/r/YmKnp3Ccu7laW3E4@alley

The safe single console lock would need to do something like:

/*
 * Safe way to take con->lock. It makes sure that @console_sem is
 * not taken and blocks anyone from taking @console_sem.
 */
void single_console_lock(struct console *con)
{
try_again:
	error = wait_event_interruptible(con->lock_wait,
			(!con->blocked &&
			 !console_kthreads_atomically_blocked()));

	/* Spurious wakeup */
	if (error)
		goto try_again;

	mutex_lock(&con->lock);

	/*
	 * Check is the console is blocked by @console_sem taken via
	 * console_lock() or if it is suspended.
	 */
	if (con->blocked) {
		mutex_unlock(@con->lock); 
		goto try_again;
	}

	/*
	 * Try to block console_trylock(). Otherwise, we are blocked by
	 * @console_set taken via console_trylock().
	 */
	if (!console_kthread_printing_tryenter()) {
		mutex_unlock(@con->lock); 
		goto try_again;
	}

	/*
	 * Eureka! We own @con->lock and both console_lock() and
	 * console_trylock() are blocked.
	 */
}

Best Regards,
Petr