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Date: Wed, 21 Oct 2015 13:11:20 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Petr Mladek <pmladek@suse.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jiri Kosina <jkosina@suse.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/spinlocks: Avoid a deadlock when someone unlock a
 zapped ticked spinlock
Message-ID: <20151021111120.GK17308@twins.programming.kicks-ass.net>
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On Wed, Oct 21, 2015 at 11:18:09AM +0200, Petr Mladek wrote:
> There are few situations when we reinitialize (zap) ticket spinlocks. It
> typically happens when the system is going down after an error and we
> want to avoid deadlock in some important services. For example,
> zap_locks() in printk.c and ioapic_zap_locks().

So there's a few problems here. On x86 the code you patch is dead code,
x86 no longer uses ticket locks. Other archs might still.

And I entirely detest adding instructions to any lock path, be it the
utmost fast path or not, for something that will _never_ happen (on a
healthy system).

I would still very much recommend getting rid of the need for
zap_locks() in the first place.

What I did back when is punt on the whole printk buffer madness and dump
things to early_printk() without any locking.

I think that as long as the printk buffer has locks you have to accept
to loose some data when really bad stuff goes down.