linux-kernel - Re: KGDB/KDB treats WARN*() as Oops on x86 since 4.12

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Message-ID: <7d12c8e1-0389-9982-9fd8-070dc0d26bef@linaro.org>
Date:   Fri, 13 Oct 2017 15:59:44 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Ilya Dryomov <idryomov@...il.com>,
        Jason Wessel <jason.wessel@...driver.com>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: KGDB/KDB treats WARN*() as Oops on x86 since 4.12

On 09/10/17 13:24, Ilya Dryomov wrote:
> Hi Jason,
> 
> Starting with 4.12, WARN*() is implemented with ud0, generating an
> Invalid Opcode exception.  KGDB/KDB gets entered as if it were an Oops,
> making KGDB/KDB rather hard to use, particularly on testing kernels.
> 
> Alexander posted a fix a while back, but Peter seems to be waiting for
> your ack.  Could you please weigh in?
> 
>    [PATCH] x86/debug: Handle warnings before the notifier chain
>    https://patchwork.kernel.org/patch/9859065/

Hmnnn... IIRC arm64 code has been also been blocked for a couple of 
releases whilst Will D. waited for an ack that never came.

My own reading of the code is that the patch in question restores the 
status quo, that there will still be mechanisms to provoke entry to 
kdb/kgdb during a warning (breakpoint on __warn, engage panic_on_warn, 
etc) and that these are not obviously recursive[1].

Put another way I'm happy to dig the patch out of my mail archive and 
throw in an Acked-By: but since I have no official role within kdb/kgdb 
(I'm just an interested bystander) it might not be enough for Peter.

Daniel.

[1] I'm not a huge x86 expert so correct me if I am wrong but I think
     its ok for us to trap here providing its for a different reason.