linux-kernel - Re: [PATCH] kgdb: Fix spurious true from in_dbg

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Date:   Thu, 7 May 2020 15:14:53 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Will Deacon <will@...nel.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        patches@...aro.org
Subject: Re: [PATCH] kgdb: Fix spurious true from in_dbg_master()

On Thu, May 07, 2020 at 09:39:30AM +0100, Will Deacon wrote:
> On Wed, May 06, 2020 at 05:42:23PM +0100, Daniel Thompson wrote:
> > Currently there is a small window where a badly timed migration could
> > cause in_dbg_master() to spuriously return true. Specifically if we
> > migrate to a new core after reading the processor id and the previous
> > core takes a breakpoint then we will evaluate true if we read
> > kgdb_active before we get the IPI to bring us to halt.
> > 
> > Fix this by checking irqs_disabled() first. Interrupts are always
> > disabled when we are executing the kgdb trap so this is an acceptable
> > prerequisite. This also allows us to replace raw_smp_processor_id()
> > with smp_processor_id() since the short circuit logic will prevent
> > warnings from PREEMPT_DEBUG.
> > 
> > Fixes: dcc7871128e9 ("kgdb: core changes to support kdb")
> > Suggested-by: Will Deacon <will@...nel.org>
> > Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> > ---
> >  include/linux/kgdb.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> > index b072aeb1fd78..4d6fe87fd38f 100644
> > --- a/include/linux/kgdb.h
> > +++ b/include/linux/kgdb.h
> > @@ -323,7 +323,7 @@ extern void gdbstub_exit(int status);
> >  extern int			kgdb_single_step;
> >  extern atomic_t			kgdb_active;
> >  #define in_dbg_master() \
> > -	(raw_smp_processor_id() == atomic_read(&kgdb_active))
> > +	(irqs_disabled() && (smp_processor_id() == atomic_read(&kgdb_active)))
> >  extern bool dbg_is_early;
> >  extern void __init dbg_late_init(void);
> >  extern void kgdb_panic(const char *msg);
> 
> Cheers, Daniel. I assume you'll route this one via the kgdb tree? I can
> live with the "small window" in the arm64 for-next/core branch ;)

Yes. I'll get this one applied very soon (thanks for Doug for the quick
review).


Daniel.