linux-kernel - Re: [PATCH PREEMPT RT] rt-mutex: fix deadlock in device mapper

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Message-ID: <alpine.LRH.2.02.1711211450410.10766@file01.intranet.prod.int.rdu2.redhat.com>
Date:   Tue, 21 Nov 2017 14:56:43 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Mike Galbraith <efault@....de>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH PREEMPT RT] rt-mutex: fix deadlock in device mapper



On Tue, 21 Nov 2017, Mike Galbraith wrote:

> On Tue, 2017-11-21 at 11:11 -0500, Mikulas Patocka wrote:
> > 
> > So, drop the spinlock unplugging and leave only mutex unplugging, 
> > reproduce the deadlock and send the stacktraces.
> 
> Nah, I reproduced it five years ago.  Is any of that relevant today?
>  Damned if I know.  Your report was the first I've noticed since.
> 
> 	-Mike

If we don't have any reason why it is needed to unplug block requests when 
a spinlock is taken - so let's not do this.

If you add code to the spinlock path and you don't know why, it is cargo 
cult programming.

Mikulas