linux-kernel - Re: [RFC][PATCHv4 6/6] printk: remove zap

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Date:   Thu, 1 Dec 2016 21:50:45 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Laura Abbott <labbott@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv4 6/6] printk: remove zap_locks() function

On (11/25/16 16:01), Petr Mladek wrote:
[..]
> > apart from detecting and reporting printk recursions, that code also
> > used to zap_lockc() in case of panic. However, zap_locks() does not
>        	          ^
> 
> s/zap_lockc/zap_locks/
> 
> > look to be needed anymore:
> > 
> > 1) Since commit 08d78658f393 ("panic: release stale console lock to
> >    always get the logbuf printed out") panic flushing of `logbuf' to
> >    console ignores the state of `console_sem' by doing
> >    	panic()
> > 		console_trylock();
> > 		console_unlock();
> > 
> > 2) Since commit cf9b1106c81c ("printk/nmi: flush NMI messages on the
> >    system panic") panic attempts to zap the `logbuf_lock' spin_lock to
> >    successfully flush nmi messages to `logbuf'.
> 
> Note that the same code is newly used to flush also the printk_safe
> per-CPU buffers. It means that logbuf_lock is zapped also when
> flushing these new buffers.


not quite. there are several checks that would prevent multiple logbuf_lock
re-inits:


        if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
                if (num_online_cpus() > 1)
                        return;

                debug_locks_off();
                raw_spin_lock_init(&logbuf_lock);
        }


	-ss