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Date: Fri, 25 Nov 2016 16:17:24 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>, Calvin Owens <calvinowens@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Laura Abbott <labbott@redhat.com>, Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [RFC][PATCHv4 6/6] printk: remove zap_locks() function
Message-ID: <20161125151724.GO3092@twins.programming.kicks-ass.net>
References: <20161027154933.1211-1-sergey.senozhatsky@gmail.com>
 <20161027154933.1211-7-sergey.senozhatsky@gmail.com>
 <20161125150113.GJ24103@pathway.suse.cz>
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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.