linux-kernel - Re: [PATCH] x86/spinlocks: Avoid a deadlock when someone unlock a zapped ticked spinlock

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Message-ID: <20151021111120.GK17308@twins.programming.kicks-ass.net>
Date:	Wed, 21 Oct 2015 13:11:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/spinlocks: Avoid a deadlock when someone unlock a
 zapped ticked spinlock

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.
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