linux-kernel - Re: [PATCH] printk: inject caller information into the body of message

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Message-ID: <3b378c7d-c613-4a8d-67f8-946fac8ad0b0@i-love.sakura.ne.jp>
Date:   Fri, 28 Sep 2018 20:01:08 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        kbuild test robot <fengguang.wu@...el.com>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] printk: inject caller information into the body of
 message

On 2018/09/28 18:09, Sergey Senozhatsky wrote:
> On (09/24/18 17:11), Tetsuo Handa wrote:
>> The reason of using statically preallocated global buffers is that I think
>> that it is inconvenient for KERN_CONT users to calculate necessary bytes
>> only for avoiding message truncation. The pr_line might be passed to deep
>> into the callchain and adjusting buffer size whenever the content's possible
>> max length changes is as much painful as changing printk() to accept only
>> one "const char *" argument. Even if we guarantee that any context can
>> allocate buffer from kernel stack, we cannot guarantee that many concurrent
>> printk() won't trigger lockup. Thus, I think that trying to allocate from
>> finite static buffers with a fallback to unbuffered printk() upon failure
>> is sufficient.
> 
> Yes, this makes sense. At the same time we can keep pr_line buffer
> in .bss
> 
> 	static char buffer[1024];
> 	static DEFINE_PR_LINE_BUF(..., buffer);
> 
> just like you have already mentioned. But that's going to require a
> case-by-case handling; so a big list of printk buffers is a simpler
> option. Fallback, tho, can be painful. On a system with 1024 CPUs can
> one have more than 16 concurrent cont printks? If the answer is yes,
> then we are looking at the same broken cont output as before.

I'm OK with making "16" configurable (at kernel configuration and/or
at kernel boot like log_buf_len= kernel command line parameter).

We could even allow each "struct task_struct" to have corresponding
"struct printk_buffer". But if there are such many concurrent callers,
the printk() would have already locked up the system to death. ;-)