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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>,
	Jiri Slaby <jslaby@suse.com>, Andreas Mohr <andi@lisas.de>,
	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?
Date: Tue, 5 Sep 2017 10:54:54 -0400	[thread overview]
Message-ID: <20170905105454.29160c5e@gandalf.local.home> (raw)
In-Reply-To: <20170904052246.GB28534@jagdpanzerIV.localdomain>

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

  parent reply	other threads:[~2017-09-05 14:54 UTC|newest]

Thread overview: 94+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-15  2:56 [RFC][PATCHv5 00/13] printk: introduce printing kernel thread Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 01/13] printk: move printk_pending out of per-cpu Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 02/13] printk: introduce printing kernel thread Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 03/13] printk: add sync printk_emergency API Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 04/13] printk: add enforce_emergency parameter Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 05/13] printk: enable printk offloading Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 06/13] printk: register PM notifier Sergey Senozhatsky
2017-08-15 11:51   ` Rafael J. Wysocki
2017-08-16  7:31     ` Sergey Senozhatsky
2017-08-16 12:58       ` Rafael J. Wysocki
2017-08-17  5:55         ` Sergey Senozhatsky
2017-08-17 15:43           ` Rafael J. Wysocki
2017-08-17 23:19             ` Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 07/13] printk: register syscore notifier Sergey Senozhatsky
2017-08-15 11:56   ` Rafael J. Wysocki
2017-08-16  6:55     ` Sergey Senozhatsky
2017-08-16 12:59       ` Rafael J. Wysocki
2017-08-15  2:56 ` [RFC][PATCHv5 08/13] printk: set watchdog_thresh as maximum value for atomic_print_limit Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 09/13] printk: add auto-emergency enforcement mechanism Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 10/13] printk: force printk_kthread to offload printing Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 11/13] printk: always offload printing from user-space processes Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 12/13] printk: do not cond_resched() when we can offload Sergey Senozhatsky
2017-08-15  2:56 ` [RFC][PATCHv5 13/13] printk: move offloading logic to per-cpu Sergey Senozhatsky
2017-08-23  8:33 ` [RFC][PATCHv5 00/13] printk: introduce printing kernel thread Sergey Senozhatsky
2017-08-28  9:05 ` printk: what is going on with additional newlines? Pavel Machek
2017-08-28 10:28   ` Sergey Senozhatsky
2017-08-28 12:21     ` Sergey Senozhatsky
2017-08-28 12:38       ` Sergey Senozhatsky
2017-08-28 12:46       ` Pavel Machek
2017-08-29 13:40         ` Sergey Senozhatsky
2017-08-29 16:37           ` Joe Perches
2017-08-29 17:00           ` Linus Torvalds
2017-08-29 17:12             ` Linus Torvalds
2017-08-29 20:41               ` Tetsuo Handa
2017-08-29 20:51                 ` Linus Torvalds
2017-09-02  6:12                   ` Tetsuo Handa
2017-09-02 17:06                     ` Linus Torvalds
2017-08-29 23:50               ` Steven Rostedt
2017-08-29 23:59                 ` Linus Torvalds
2017-08-30  1:03                 ` Sergey Senozhatsky
2017-08-30  1:10                   ` Steven Rostedt
2017-08-30  1:51                     ` Sergey Senozhatsky
2017-08-30  1:52                     ` Joe Perches
2017-08-30  2:25                       ` Sergey Senozhatsky
2017-08-30  2:31                         ` Joe Perches
2017-08-30  2:47                           ` Sergey Senozhatsky
2017-08-30  2:58                             ` Joe Perches
2017-08-30  5:37                               ` Sergey Senozhatsky
2017-09-08 10:18                                 ` Pavel Machek
2017-09-05  9:44                             ` Petr Mladek
2017-09-05  9:59                               ` Sergey Senozhatsky
2017-09-05 12:21                                 ` Petr Mladek
2017-09-05 12:35                                   ` Tetsuo Handa
2017-09-05 14:18                                     ` Sergey Senozhatsky
2017-09-05 13:42                                   ` Sergey Senozhatsky
2017-09-06  7:55                                     ` Petr Mladek
2017-09-17  6:26                                       ` Sergey Senozhatsky
2017-09-17  9:27                                         ` Sergey Senozhatsky
2017-09-17 15:35                                         ` Linus Torvalds
2017-09-18  0:46                                           ` Sergey Senozhatsky
2017-09-18  2:22                                             ` Joe Perches
2017-09-18  2:41                                               ` Sergey Senozhatsky
2017-09-18  2:45                                                 ` Joe Perches
2017-09-18  2:55                                                   ` Sergey Senozhatsky
2017-09-18  3:07                                                     ` Joe Perches
2017-09-18  4:42                                                       ` Sergey Senozhatsky
2017-09-01 13:19                     ` Sergey Senozhatsky
2017-09-01 17:32                       ` Joe Perches
2017-09-01 20:21                         ` Linus Torvalds
2017-09-04  5:22                           ` Sergey Senozhatsky
2017-09-04  5:41                             ` Joe Perches
2017-09-05 14:54                             ` Steven Rostedt [this message]
2017-09-06  2:14                               ` Sergey Senozhatsky
2017-09-06  2:36                               ` Linus Torvalds
2017-09-04  4:30                         ` Sergey Senozhatsky
2017-09-04  5:24                           ` Sergey Senozhatsky
2017-08-29 17:33             ` Sergey Senozhatsky
2017-08-29 17:52               ` Linus Torvalds
2017-08-29 18:09                 ` Joe Perches
2017-08-30  1:07                   ` Sergey Senozhatsky
2017-08-30  0:58                 ` Sergey Senozhatsky
2017-08-29 16:48   ` Linus Torvalds
2017-08-29 17:10     ` Joe Perches
2017-08-29 17:20       ` Linus Torvalds
2017-08-29 17:33         ` Joe Perches
2017-08-29 17:36           ` Linus Torvalds
2017-08-29 17:48             ` Joe Perches
2017-08-29 20:24     ` Pavel Machek
2017-09-01  1:40       ` Sergey Senozhatsky
2017-09-01  2:04         ` Joe Perches
2017-09-01  6:59           ` Pavel Machek
2017-09-01  7:23             ` Joe Perches
2017-09-01  7:29         ` Pavel Machek
2017-09-01 11:13           ` Steven Rostedt

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