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

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Message-ID: <20171110061550.rgxdy7tmh4ksf4mt@gmail.com>
Date:   Fri, 10 Nov 2017 07:15:50 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Ilya Dryomov <idryomov@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Daniel Thompson <daniel.thompson@...aro.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


* Ilya Dryomov <idryomov@...il.com> wrote:

> On Fri, Oct 13, 2017 at 4:59 PM, Daniel Thompson
> <daniel.thompson@...aro.org> wrote:
> > 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.
> 
> Hi Peter, Ingo,
> 
> Could you please consider taking Alexander's patch for 4.15?  Jason
> never replied to any of our pings and hasn't been actively involved
> with kgdb recently.  In the meantime, this regression makes running
> e.g. xfstests runs with kgdb enabled pretty much impossible.

Ok, agreed, I picked the fix up into tip:x86/urgent, with a -stable backporting 
tag, and will try to get it to Linus for v4.15 (it will also get backported to 
v4.14 which is affected as well).

Thanks,

	Ingo