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

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Message-ID: <20200507083929.GC28215@willie-the-truck>
Date:   Thu, 7 May 2020 09:39:30 +0100
From:   Will Deacon <will@...nel.org>
To:     Daniel Thompson <daniel.thompson@...aro.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 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 ;)

Will