linux-kernel - Re: [RFC][PATCHv5 7/7] printk: remove zap

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Date:   Tue, 13 Dec 2016 10:26:06 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv5 7/7] printk: remove zap_locks() function

On (12/12/16 17:37), Petr Mladek wrote:
> On Thu 2016-12-01 22:55:46, Sergey Senozhatsky wrote:
> > We use printk-safe now which makes printk-recursion detection code
> > in vprintk_emit() unreachable. The tricky thing here is that, apart
> > from detecting and reporting printk recursions, that code also used
> > to zap_locks() in case of panic() from the same CPU. However,
> > zap_locks() does not 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'.
> > 
> > Basically, it seems that we either already do what zap_locks() used to
> > do but in other places or we ignore the state of the lock. The only
> > reaming difference is that we don't re-init the console semaphore in
> > printk_safe_flush_on_panic(), but this is not necessary because we
> > don't call console drivers from printk_safe_flush_on_panic() due to
> > the fact that we are using a deferred printk() version (as was
> > suggested by Petr Mladek).
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> I like this patch. The code is unused after protecting the section
> by printk_safe_enter()/exit(). And also the panic mode is handled
> by the commits mentioned above.

and the scenario that code tried to address was super-tiny.
basically, a recursion from vscnprintf()/log_output() every 30
seconds (zap_locks() timeout):

	if (unlikely(logbuf_cpu == this_cpu)) {
		zap_locks();
	}

	raw_spin_lock(&logbuf_lock);
	logbuf_cpu = this_cpu;

	text_len = vscnprintf(text, sizeof(textbuf), fmt, args);
	kern_level = printk_get_level(text));
	log_output(facility, level, lflags, dict, dictlen, text, text_len);

	logbuf_cpu = UINT_MAX;
	raw_spin_unlock(&logbuf_lock);

now we cover a much bigger scope.


> Reviewed-by: Petr Mladek <pmladek@...e.com>

thanks.

	-ss