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

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Date:   Thu, 1 Dec 2016 11:34:42 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        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>,
        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 16:17), Peter Zijlstra wrote:
> On Fri, Nov 25, 2016 at 04:01:13PM +0100, Petr Mladek wrote:
> > On Fri 2016-10-28 00:49:33, Sergey Senozhatsky wrote:
> > > 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.
> > 
> 
> Note that (raw_)spin_lock_init() as done here and in
> printk_nmi_flush_on_panic() can wreck the lock state and doesn't ensure
> a subsequent spin_lock() of said lock will actually work.
> 
> The very best solution is to simply ignore the lock in panic situations
> rather than trying to wreck it.

do you mean that we can enterily drop the spin_lock_init()? or is there
something else? spin_lock_init() either does not improve anything or let
us to, at least, move the messages from per-CPU buffers to the logbuf. so
it's not like it does some damage, and it can help sometimes. though I
agree that a) we have the messages in the memory already and b) logbuf_lock
is not the one&only troubling lock.

	-ss