Re: [PATCH printk v7 24/35] printk: nbcon: Flush new records on device_release()

[John Ogness <john.ogness@linutronix.de>]

On 2024-08-05, Petr Mladek <pmladek@suse.com> wrote:
>> +	/*
>> +	 * This context must flush any new records added while the console
>> +	 * was locked. The console_srcu_read_lock must be taken to ensure
>> +	 * the console is usable throughout flushing.
>> +	 */
>> +	cookie = console_srcu_read_lock();
>> +	if (console_is_usable(con, console_srcu_read_flags(con)) &&
>> +	    prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
>> +		if (!have_boot_console) {
>> +			__nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb));
>> +		} else if (!is_printk_legacy_deferred()) {
>> +			if (console_trylock())
>> +				console_unlock();
>
> nbcon_device_release() is going to be called in uart_port_unlock*()
> still under the port->lock.
>
> => It smells with a potential deadlock. The console_flush_all() in
>    console_unlock() might want to flush this console under the
>    port->lock as well.
>
>    And it almost happens because nbcon_legacy_emit_next_record()
>    might eventually take con->device_lock() when called in
>    a task context.
>
>    It will not happen here because this code uses console_trylock()
>    which would set @console_may_schedule to false.

Exactly. That is an important point. We must never try to invoke the
write_thread() callback while holding a spinlock.

> Anyway, it would look more safe when the flush was done after releasing
> the port->lock.

Even then we could never invoke the write_thread() callback because the
caller may be holding other spinlocks. If we cannot safely call
console_lock(), we cannot take the device lock. The atomic callback must
be used and that means the port lock cannot be involved in the
console_trylock().

John