linux-kernel - Re: [PATCH v3] kdb: Make memory allocations more robust

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Date:   Thu, 28 Jan 2021 10:41:12 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "# 4.0+" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] kdb: Make memory allocations more robust

On Mon, 25 Jan 2021 at 13:48, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Fri, Jan 22, 2021 at 09:25:44AM -0800, Doug Anderson wrote:
> > Hi,
> >
> > On Fri, Jan 22, 2021 at 3:06 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> > >
> > > Currently kdb uses in_interrupt() to determine whether its library
> > > code has been called from the kgdb trap handler or from a saner calling
> > > context such as driver init.  This approach is broken because
> > > in_interrupt() alone isn't able to determine kgdb trap handler entry from
> > > normal task context. This can happen during normal use of basic features
> > > such as breakpoints and can also be trivially reproduced using:
> > > echo g > /proc/sysrq-trigger
> >
> > I guess an alternative to your patch is to fully eliminate GFP_KDB.
> > It always strikes me as a sub-optimal design to choose between
> > GFP_ATOMIC and GFP_KERNEL like this.  Presumably others must agree
> > because otherwise I'd expect that the overall kernel would have
> > something like "GFP_AUTOMATIC"?
> >
> > It doesn't feel like it'd be that hard to do something more explicit.
> > From a quick glance:
> >
> > * I think kdb_defcmd() and kdb_defcmd2() are always called in response
> > to a user typing something on the kdb command line.  Those should
> > always be GFP_ATOMIC, right?
>
> No. I'm afraid not. The kdb parser is also used to execute
> kernel/debug/kdb/kdb_cmds as part of the kdb initialization. This
> initialization happens from the init calls rather than from the kgdb
> trap handler code.
>
> When I first looked at Sumit's patch I had a similar reaction to you
> but, whilst it is clearly it's not impossible to pass flags into the
> kdb parser and all its subcommands, I concluded that GFP_KDB is a
> better approach.
>
> BTW the reason I insisted on getting rid of the in_atomic() was to make
> it clear that GFP_KDB discriminates between exactly two calling contexts
> (normal and kgdb trap handler). I was didn't want any hints that imply
> GFP_KDB is a (broken) implementation of something like GFP_AUTOMATIC!
>

Ah, I see the reasoning to keep GFP_KDB. So we don't need any further
refactoring and can go ahead with this patch only.

-Sumit

>
> Daniel.