linux-kernel - Re: [RFC PATCH v1 08/25] printk: add ring buffer and kthread

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Date:   Fri, 8 Mar 2019 10:31:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 08/25] printk: add ring buffer and kthread

On (03/07/19 13:06), John Ogness wrote:
> On 2019-03-04, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:
> >> +	/* the printk kthread never exits */
> >> +	for (;;) {
> >> +		ret = prb_iter_wait_next(&iter, buf,
> >> +					 PRINTK_RECORD_MAX, &master_seq);
> >> +		if (ret == -ERESTARTSYS) {
> >> +			continue;
> >> +		} else if (ret < 0) {
> >> +			/* iterator invalid, start over */
> >> +			prb_iter_init(&iter, &printk_rb, NULL);
> >> +			continue;
> >> +		}
> >> +
> >> +		msg = (struct printk_log *)buf;
> >> +		format_text(msg, master_seq, ext_text, &ext_len, text,
> >> +			    &len, printk_time);
> >> +
> >> +		console_lock();
> >> +		if (len > 0 || ext_len > 0) {
> >> +			call_console_drivers(ext_text, ext_len, text, len);
> >> +			boot_delay_msec(msg->level);
> >> +			printk_delay();
> >> +		}
> >> +		console_unlock();
> >> +	}
> >
> > This, theoretically, creates a whole new world of possibilities for
> > console drivers. Now they can do GFP_KERNEL allocations and stall
> > printk_kthread during OOM; or they can explicitly reschedule from
> > ->write() callback (via console_conditional_schedule()) because
> > console_lock() sets console_may_schedule.
> 
> This was the intention.

This can stall the entire printing pipeline

OOM -> printk_kthread() -> console_lock() -> con_foo() -> kmalloc(GFP_KERNEL) -> OOM

> Although, as I mentioned in a previous response[0], perhaps we should
> not loosen the requirements on write().

Right. Console drivers better stay restricted; very restricted.

> It is exactly that disable_preempt() that is so harmful for realtime tasks.

I'll reply in another email (later today, or tomorrow).

	-ss