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

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Date:   Thu, 8 Nov 2018 13:45:10 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        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>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep
 messages.

On (11/07/18 16:19), Petr Mladek wrote:
> > syzbot is sometimes getting mixed output like below due to concurrent
> > printk(). Mitigate such output by using line-buffered printk() API.
> > 
> > @@ -2421,18 +2458,20 @@ static void check_chain_key(struct task_struct *curr)
> >  print_usage_bug_scenario(struct held_lock *lock)
> >  {
> >  	struct lock_class *class = hlock_class(lock);
> > +	struct printk_buffer *buf = get_printk_buffer();
> >  
> >  	printk(" Possible unsafe locking scenario:\n\n");
> >  	printk("       CPU0\n");
> >  	printk("       ----\n");
> > -	printk("  lock(");
> > -	__print_lock_name(class);
> > -	printk(KERN_CONT ");\n");
> > +	printk_buffered(buf, "  lock(");
> > +	__print_lock_name(class, buf);
> > +	printk_buffered(buf, ");\n");
> >  	printk("  <Interrupt>\n");
> > -	printk("    lock(");
> > -	__print_lock_name(class);
> > -	printk(KERN_CONT ");\n");
> > +	printk_buffered(buf, "    lock(");
> > +	__print_lock_name(class, buf);
> > +	printk_buffered(buf, ");\n");
> >  	printk("\n *** DEADLOCK ***\n\n");
> > +	put_printk_buffer(buf);
> >  }
> >  
> >  static int
> 
> I really hope that the maze of pr_cont() calls in lockdep.c is the most
> complicated one that we would meet.

Hmm... Yes, buffered/seq_buf printk looks like a hard-to-use API,
when it comes to real world cases like this.

So... here is a random and wild idea.

We actually already have an easy-to-use buffered printk. And it's per-CPU.
And it makes all printk-s on this CPU to behave like as if they were called
on UP system. And it cures pr_cont(). And it doesn't require anyone to learn
any new printk API names. And it doesn't require any additional maintenance
work. And it doesn't require any printk->buffered_printk conversions. And
it's already in the kernel. And we gave it a name. And it's printk_safe.

a) lockdep reporting path should be atomic. And it's not a hot path,
   so local_irq_save/local_irq_restore will not cause a lot of trouble
   there probably.

b) We already have some lockdep reports coming via printk_safe.
   All those
	printk->console_driver->scheduler->lockdep
	printk->console_driver->timekeeping->lockdep
	etc.

   came via printk_safe path. So it's not a complete novelty.

c) printk_safe sections can nest.

d) No premature flushes. Everything looks the way it was supposed to
   look.

e) There are no out-of-line printk-s. We keep the actual order of events.

f) We flush it on panic.

g) Low maintenance costs.

So, can we just do the following? /* a sketch */

lockdep.c
	printk_safe_enter_irqsave(flags);
	lockdep_report();
	printk_safe_exit_irqrestore(flags);

	-ss