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Date: Tue, 5 Sep 2017 10:54:54 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Joe Perches <joe@perches.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Pavel Machek <pavel@ucw.cz>, Petr Mladek <pmladek@suse.com>,
        Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
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        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: printk: what is going on with additional newlines?
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On Mon, 4 Sep 2017 14:22:46 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:

> like I said in another email, printk-safe buffer
> is per-CPU and is also used for actual printk-safe, hence it must be
> used with local IRQs disabled when we "borrow" the buffer for pr_line
> (disabled preemption is not enough due to possible IRQ printk-safe
> print out). this can be a bit annoying.

You can do what I did with trace_printk(). I have a buffer per context.
Then you only need to use preempt_disable() to do the print. That is,
trace_printk() has 4 buffers:

 1. Normal context
 2. softirq context
 3. irq context
 4. NMI context

It determines which context it is in, disables preemption, and uses the
corresponding buffer. This way I don't need to worry about being
preempted by an interrupt or NMI.

Grant it, it does make the memory needed 4x bigger.

I have an array of 4 buffers, and the following code:

static char *get_trace_buf(void)
{
	struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);

	if (!buffer || buffer->nesting >= 4)
		return NULL;

	return &buffer->buffer[buffer->nesting++][0];
}

Hmm, I probably need to add a "barrier()" before the return, or use a
this_cpu_inc() on nesting. As long as the nesting variable is updated
before the return of the buffer being used, then everything is fine.
Because we have:

static void put_trace_buf(void)
{
	this_cpu_dec(trace_percpu_buffer->nesting);
}

And anything that preempts this call will have returned it back to its
original state before returning.

-- Steve