linux-kernel - Re: [RFC PATCH 2/2] locking/spinlock/debug: Add checks for kgdb trap safety

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Message-ID: <20200522163504.GX317569@hirez.programming.kicks-ass.net>
Date:   Fri, 22 May 2020 18:35:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     sumit.garg@...aro.org, jason.wessel@...driver.com,
        dianders@...omium.org, kgdb-bugreport@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, patches@...aro.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC PATCH 2/2] locking/spinlock/debug: Add checks for kgdb trap
 safety

On Fri, May 22, 2020 at 03:55:10PM +0100, Daniel Thompson wrote:
> In general it is not safe to call spin_lock() whilst executing in the
> kgdb trap handler. The trap can be entered from all sorts of execution
> context (NMI, IRQ, irqs disabled, etc) and the kgdb/kdb needs to be
> as resillient as possible.
> 
> Currently it is difficult to spot mistakes in the kgdb/kdb logic
> (especially so for kdb because it uses more kernel features than
> pure-kgdb). Let's provide a means to bring attention to deadlock
> risks in the debug code.

I really dislike this thing. Also, commit:

  f6f48e180404 ("lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions")

should be able to trigger here when the kgdb traps are marked as NMI.
x86 will soon have that.