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SubjectRe: [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

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