linux-kernel - Re: [PATCH] arm64: kcsan: Fix kcsan test

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Message-ID: <YmfhHuPDilwR/Wgp@elver.google.com>
Date:   Tue, 26 Apr 2022 14:10:06 +0200
From:   Marco Elver <elver@...gle.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com
Subject: Re: [PATCH] arm64: kcsan: Fix kcsan test_barrier fail and panic

On Tue, Apr 26, 2022 at 08:17AM +0000, Kefeng Wang wrote:
> As "kcsan: Support detecting a subset of missing memory barriers"
> introduced KCSAN_STRICT which make kcsan detects more missing memory
> barrier, but arm64 don't have KCSAN instrumentation for barriers, so
> the new selftest test_barrier() will fail, then panic.

Thanks for fixing this - did kcsan_test module pass as well?

> Let's prefix all barriers with __ on arm64, as asm-generic/barriers.h
> defined the final instrumented version of these barriers, which will
> fix the above issues.
> 
> Fixes: dd03762ab608 ("arm64: Enable KCSAN")
> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> ---
>  arch/arm64/include/asm/barrier.h | 12 ++++++------
>  include/asm-generic/barrier.h    |  4 ++++
>  2 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 62217be36217..9760a8d4ed0a 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -46,13 +46,13 @@
>  #define pmr_sync()	do {} while (0)
>  #endif
>  
> -#define mb()		dsb(sy)
> -#define rmb()		dsb(ld)
> -#define wmb()		dsb(st)
> +#define __mb()		dsb(sy)
> +#define __rmb()		dsb(ld)
> +#define __wmb()		dsb(st)
>  
> -#define dma_mb()	dmb(osh)
> -#define dma_rmb()	dmb(oshld)
> -#define dma_wmb()	dmb(oshst)
> +#define __dma_mb()	dmb(osh)
> +#define __dma_rmb()	dmb(oshld)
> +#define __dma_wmb()	dmb(oshst)
>  
>  #define io_stop_wc()	dgh()
>  
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index fd7e8fbaeef1..18863c50e9ce 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -38,6 +38,10 @@
>  #define wmb()	do { kcsan_wmb(); __wmb(); } while (0)
>  #endif
>  
> +#ifdef __dma_mb
> +#define dma_mb()	do { kcsan_mb(); __dma_mb(); } while (0)
> +#endif
> +

So it looks like arm64 is the only arch that defines dma_mb(). By adding
it to asm-generic, we'd almost be encouraging other architectures to add
it, which I don't know we want.

Documentation/memory-barriers.txt doesn't mention dma_mb() either - so
perhaps dma_mb() doesn't belong in asm-generic/barrier.h, and you could
only change arm64's definition of dma_mb() to add the kcsan_mb().

Preferences? Maybe arch64 maintainers have more background on why arm64
is an anomaly here.