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Message-ID: <ZrDwZfGriZSxmjnp@pathway.suse.cz>
Date: Mon, 5 Aug 2024 17:31:49 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v7 24/35] printk: nbcon: Flush new records on
device_release()
On Sun 2024-08-04 02:57:27, John Ogness wrote:
> There may be new records that were added while a driver was
> holding the nbcon context for non-printing purposes. These
> new records must be flushed by the nbcon_device_release()
> context because no other context will do it.
>
> If boot consoles are registered, the legacy loop is used
> (either direct or per irq_work) to handle the flushing.
>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
It makes some sense and seems to work. I do not know how to
make it better.
Reviewed-by: Petr Mladek <pmladek@...e.com>
But it makes me nervous a bit, see below:
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1326,10 +1326,30 @@ EXPORT_SYMBOL_GPL(nbcon_device_try_acquire);
> void nbcon_device_release(struct console *con)
> {
> struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_device_ctxt);
> + int cookie;
>
> if (!nbcon_context_exit_unsafe(ctxt))
> return;
>
> nbcon_context_release(ctxt);
> +
> + /*
> + * 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.
Anyway, it would look more safe when the flush was done after releasing
the port->lock.
I still have to think about this.
> + } else {
> + printk_trigger_flush();
> + }
> + }
> + console_srcu_read_unlock(cookie);
> }
> EXPORT_SYMBOL_GPL(nbcon_device_release);
Best Regards,
Petr
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