linux-kernel - Re: [PATCH v2] arm64: cacheflush: Fix KGDB trap detection

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Date:   Mon, 4 May 2020 21:48:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Douglas Anderson <dianders@...omium.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        patches@...aro.org
Subject: Re: [PATCH v2] arm64: cacheflush: Fix KGDB trap detection

On Mon, May 04, 2020 at 06:05:18PM +0100, Daniel Thompson wrote:
> flush_icache_range() contains a bodge to avoid issuing IPIs when the kgdb
> trap handler is running because issuing IPIs is unsafe (and not needed)
> in this execution context. However the current test, based on
> kgdb_connected is flawed: it both over-matches and under-matches.
> 
> The over match occurs because kgdb_connected is set when gdb attaches
> to the stub and remains set during normal running. This is relatively
> harmelss because in almost all cases irq_disabled() will be false.
> 
> The under match is more serious. When kdb is used instead of kgdb to access
> the debugger then kgdb_connected is not set in all the places that the
> debug core updates sw breakpoints (and hence flushes the icache). This
> can lead to deadlock.
> 
> Fix by replacing the ad-hoc check with the proper kgdb macro. This also
> allows us to drop the #ifdef wrapper.
> 
> Fixes: 3b8c9f1cdfc5 ("arm64: IPI each CPU after invalidating the I-cache for kernel mappings")
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>
> ---
> 
> Notes:
>     v2: Improve the commit message based based on feedback from Doug
>         Anderson
> 
>  arch/arm64/include/asm/cacheflush.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cacheflush.h b/arch/arm64/include/asm/cacheflush.h
> index e6cca3d4acf7..ce50c1f1f1ea 100644
> --- a/arch/arm64/include/asm/cacheflush.h
> +++ b/arch/arm64/include/asm/cacheflush.h
> @@ -79,7 +79,7 @@ static inline void flush_icache_range(unsigned long start, unsigned long end)
>  	 * IPI all online CPUs so that they undergo a context synchronization
>  	 * event and are forced to refetch the new instructions.
>  	 */
> -#ifdef CONFIG_KGDB
> +
>  	/*
>  	 * KGDB performs cache maintenance with interrupts disabled, so we
>  	 * will deadlock trying to IPI the secondary CPUs. In theory, we can
> @@ -89,9 +89,9 @@ static inline void flush_icache_range(unsigned long start, unsigned long end)
>  	 * the patching operation, so we don't need extra IPIs here anyway.
>  	 * In which case, add a KGDB-specific bodge and return early.
>  	 */
> -	if (kgdb_connected && irqs_disabled())
> +	if (in_dbg_master())

Does this imply that irqs are disabled?

Will