linux-kernel - Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages.

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Date:   Sat, 3 Nov 2018 11:00:10 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep
 messages.

On 2018/11/02 22:36, Peter Zijlstra wrote:
> On Fri, Nov 02, 2018 at 10:31:57PM +0900, Tetsuo Handa wrote:
>> syzbot is sometimes getting mixed output like below due to concurrent
>> printk(). Mitigate such output by using line-buffered printk() API.
>>
>>   RCU used illegally from idle CPU!
>>   rcu_scheduler_active = 2, debug_locks = 1
>>   RSP: 0018:ffffffff88007bb8 EFLAGS: 00000286
>>   RCU used illegally from extended quiescent state!
>>    ORIG_RAX: ffffffffffffff13
>>   1 lock held by swapper/1/0:
>>   RAX: dffffc0000000000 RBX: 1ffffffff1000f7b RCX: 0000000000000000
>>    #0: 
>>   RDX: 1ffffffff10237b8 RSI: 0000000000000001 RDI: ffffffff8811bdc0
>>   000000004b34587c
>>   RBP: ffffffff88007bb8 R08: ffffffff88075e00 R09: 0000000000000000
>>    (
>>   R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>>   rcu_read_lock
>>   R13: ffffffff88007c78 R14: 0000000000000000 R15: 0000000000000000
>>   ){....}
>>    arch_safe_halt arch/x86/include/asm/paravirt.h:94 [inline]
>>    default_idle+0xc2/0x410 arch/x86/kernel/process.c:498
>>   , at: trace_call_bpf+0xf8/0x640 kernel/trace/bpf_trace.c:46
> 
> WTH is that buffered aPI, and no, that breaks my earlyprintk stuff.
> 

This API is nothing but a wrapper for reducing frequency of directly
calling printk() by using snprintf() if possible. Thus, whatever your
earlyprintk stuff is, this API should not affect it.