linux-kernel - Re: [PATCH 2/2] printk: wake up klogd in vprintk

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Message-ID: <20180418150214.z7oyughldrktj6e4@pathway.suse.cz>
Date:   Wed, 18 Apr 2018 17:02:14 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] printk: wake up klogd in vprintk_emit

On Wed 2018-04-18 10:29:43, Steven Rostedt wrote:
> On Sat, 14 Apr 2018 12:01:45 +0900
> Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:
> 
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -1888,6 +1888,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> >  
> >  	printed_len = log_output(facility, level, lflags, dict, dictlen, text, text_len);
> >  
> > +	wake_up_klogd();
> >  	logbuf_unlock_irqrestore(flags);
> 
> You can't do this, because the scheduler can call printk_deferred()
> with the rq lock held, and printk_deferred() will grab the logbuf lock.
> 
> Calling wake_up_klogd() will grab the rq lock and give us a A-B<->B-A
> locking order.

wake_up_klogd() uses the lockless irq_work_queue(). So it is actually
safe.

But the name is confusing. We should rename it.

Best Regards,
Petr