linux-kernel - Re: printk() from NMI backtrace can delay a lot

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Message-ID: <20180703163948.GD391@tigerII.localdomain>
Date:   Wed, 4 Jul 2018 01:39:48 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: printk() from NMI backtrace can delay a lot

On (07/04/18 01:31), Sergey Senozhatsky wrote:
> it might be the case that we can do
> 
> 	serial_driver_handle_irq()
> 	{
> 		spin_loc(uart_port->lock);
> 
> 		.. TX / RX
> 
> 		spin_unlock(uart_port->lock);
> 
> 		tty_flip_buffer_push(uart_port->tty_port);
> 	}
> 
> This should break this chain
> 
> 	uart_port->lock -> pool->lock   // -> sheduler/etc.
> 
> 
> Can we do it? What am I missing?

Hmm, and we actually have some serial drivers that do tty_filp_buffer_push()
outside of uart_port->lock. For instance, drivers/tty/serial/efm32-uart.c
irqreturn_t efm32_uart_rxirq().

	-ss