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

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Message-ID: <b725c54e-a5b7-12fe-8269-8beccc4c88ce@i-love.sakura.ne.jp>
Date:   Tue, 6 Nov 2018 18:56:03 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 2018/11/06 17:38, Petr Mladek wrote:
> If you would want to avoid buffering, you could set the number
> of buffers to zero. Then it would always fallback to
> the direct printk().

  1 lock held by swapper/1/0:
   #0: 
   (
  rcu_read_lock
  ){....}
  , at: trace_call_bpf+0xf8/0x640 kernel/trace/bpf_trace.c:46

is not welcomed and

  1 lock held by swapper/1/0:
   #0:  (rcu_read_lock){....}, at: trace_call_bpf+0xf8/0x640 kernel/trace/bpf_trace.c:46

is welcomed.

If you want to avoid fallback to direct printk(), please allocate on-stack
buffer with appropriate size. Since lockdep splat may happen when kernel
stack is already tight, blindly allocating large buffer on the stack is
not good.