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

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Date:   Thu, 1 Dec 2016 11:18:04 +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>,
        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:
> On Fri 2016-10-28 00:49:33, Sergey Senozhatsky wrote:
> > We use printk-safe now which makes printk-recursion detection code
> > in vprintk_emit() is unreachable. The tricky thing here is that,
>      		    ^^ superfluous "is"
> 
> > 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.

yes, and I'm a bit skeptical about the whole re-init logbuf_lock,
because this lock is just one of possibly many looks that can be
'locked'. the messages are already in per-CPU buffers, so they
will present in a core dump file.

but it can be done separately, not in this series.

> > 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. May be we
> > still would want to do sema_init() in printk_safe_flush_on_panic(),
> > just in case.
> 
> Very good question! I would actually suggest to use printk_deferred()
> in printk_safe_flush_on_panic() in any context. It will solve the
> problems discussed for the 4th patch of this patchset. And it will
> solve also this problem. In case of panic, we should first try to
> get all messages into the logbuffer so that they are visible in
> the crash dump. We try to push them to the console by
> console_flush_on_panic() later because it is more risky.
>
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> If we avoid calling console in printk_safe_flush_on_panic(),
> feel free to use:
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>

ok. thanks.

	-ss