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

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Message-ID: <20210122094810.6o32gzoqtwgqi5hn@maple.lan>
Date:   Fri, 22 Jan 2021 09:48:10 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net, jason.wessel@...driver.com,
        dianders@...omium.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kdb: Make memory allocations more robust

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?


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
>