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

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Message-ID: <CAFA6WYO0MFn8A-yDjThrBj_DxEpS3j6DixJX9gi49So99BZg2Q@mail.gmail.com>
Date:   Fri, 22 Jan 2021 15:46:25 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kdb: Make memory allocations more robust

On Fri, 22 Jan 2021 at 15:18, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Fri, Jan 22, 2021 at 03:08:31PM +0530, Sumit Garg wrote:
> > Currently kdb uses in_interrupt() to determine whether it's 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 via
> > normal task context such as [1].
> >
> > We can improve this by adding check for in_dbg_master() which explicitly
> > determines if we are running in debugger context. Also, use in_atomic()
> > instead of in_interrupt() as the former is more appropriate to know atomic
> > context and moreover the later one is deprecated.
>
> Why do we need the in_atomic() here? Or put another way, why isn't
> in_dbg_master() sufficient?
>

Yes, you are right in_atomic() is redundant after looking at usage of
GFP_KDB. Will get rid of it in v2.

-Sumit

>
> Daniel.
>
>
> >
> > [1] $ echo g > /proc/sysrq-trigger
> >
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > ---
> >  kernel/debug/kdb/kdb_private.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/debug/kdb/kdb_private.h b/kernel/debug/kdb/kdb_private.h
> > index 7a4a181..7a9ebd9 100644
> > --- a/kernel/debug/kdb/kdb_private.h
> > +++ b/kernel/debug/kdb/kdb_private.h
> > @@ -231,7 +231,7 @@ extern struct task_struct *kdb_curr_task(int);
> >
> >  #define kdb_task_has_cpu(p) (task_curr(p))
> >
> > -#define GFP_KDB (in_interrupt() ? GFP_ATOMIC : GFP_KERNEL)
> > +#define GFP_KDB (in_atomic() || in_dbg_master() ? GFP_ATOMIC : GFP_KERNEL)
> >
> >  extern void *debug_kmalloc(size_t size, gfp_t flags);
> >  extern void debug_kfree(void *);
> > --
> > 2.7.4
> >