linux-kernel - Re: [PATCH] arm: perf: Mark expected switch fall-through

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Date:   Fri, 26 Jul 2019 14:08:34 +0100
From:   Will Deacon <will@...nel.org>
To:     Anders Roxell <anders.roxell@...aro.org>
Cc:     mark.rutland@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] arm: perf: Mark expected switch fall-through

On Fri, Jul 26, 2019 at 01:27:32PM +0200, Anders Roxell wrote:
> When fall-through warnings was enabled by default, d93512ef0f0e
> ("Makefile: Globally enable fall-through warning"), we could see the
> following warnings was starting to show up. However, this was originally
> introduced in commit 6ee33c2712fc ("ARM: hw_breakpoint: correct and
> simplify alignment fixup code"). Commit d968d2b801d8 ("ARM: 7497/1:
> hw_breakpoint: allow single-byte watchpoints on all addresses") was
> written with the intent to allow single-byte watchpoints on all
> addresses but forgot to move 'case 1:' down below 'case 2:'.
> 
> ../arch/arm/kernel/hw_breakpoint.c: In function ‘hw_breakpoint_arch_parse’:
> ../arch/arm/kernel/hw_breakpoint.c:609:7: warning: this statement may fall
>  through [-Wimplicit-fallthrough=]
>     if (hw->ctrl.len == ARM_BREAKPOINT_LEN_2)
>        ^
> ../arch/arm/kernel/hw_breakpoint.c:611:3: note: here
>    case 3:
>    ^~~~
> ../arch/arm/kernel/hw_breakpoint.c:613:7: warning: this statement may fall
>  through [-Wimplicit-fallthrough=]
>     if (hw->ctrl.len == ARM_BREAKPOINT_LEN_1)
>        ^
> ../arch/arm/kernel/hw_breakpoint.c:615:3: note: here
>    default:
>    ^~~~~~~
> 
> Rework so 'case 1:' are next to 'case 3:' and also add '/* Fall through
> */' so that the compiler doesn't warn about fall-through.
> 
> Cc: stable@...r.kernel.org # v3.16
> Fixes: 6ee33c2712fc ("ARM: hw_breakpoint: correct and simplify alignment fixup code")
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
> ---
>  arch/arm/kernel/hw_breakpoint.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
> index af8b8e15f589..c14d506969ba 100644
> --- a/arch/arm/kernel/hw_breakpoint.c
> +++ b/arch/arm/kernel/hw_breakpoint.c
> @@ -603,15 +603,17 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
>  	case 0:
>  		/* Aligned */
>  		break;
> -	case 1:
>  	case 2:
>  		/* Allow halfword watchpoints and breakpoints. */
>  		if (hw->ctrl.len == ARM_BREAKPOINT_LEN_2)
>  			break;
> +		/* Fall through */
> +	case 1:

Why are you moving the 'case 1:' here? AFAICT, your patch now rejects
byte-aligned watchpoints of length 2.

Will