linux-kernel - Re: [PATCH v2 2/2] io: prevent compiler reordering on the default readX() implementation

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Date:   Tue, 3 Apr 2018 11:49:25 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     arnd@...db.de, timur@...eaurora.org, sulrich@...eaurora.org,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] io: prevent compiler reordering on the default
 readX() implementation

Hi,

On Fri, Mar 30, 2018 at 11:58:13AM -0400, Sinan Kaya wrote:
> The default implementation of mapping readX() to __raw_readX() is wrong.
> readX() has stronger ordering semantics. Compiler is allowed to reorder
> __raw_readX().

Could you please specify what the compiler is potentially reordering
__raw_readX() against, and why this would be wrong?

e.g. do we care about prior normal memory accesses, subsequent normal
memory accesses, and/or other IO accesses?

I assume that the asm-generic __raw_{read,write}X() implementations are
all ordered w.r.t. each other (at least for a specific device).

Thanks,
Mark.

> In the abscence of a read barrier or when using a strongly ordered
> architecture, readX() should at least have a compiler barrier in
> it to prevent commpiler from clobbering the execution order.
> 
> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
> ---
>  include/asm-generic/io.h | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/include/asm-generic/io.h b/include/asm-generic/io.h
> index e8c2078..2554f15 100644
> --- a/include/asm-generic/io.h
> +++ b/include/asm-generic/io.h
> @@ -110,7 +110,12 @@ static inline void __raw_writeq(u64 value, volatile void __iomem *addr)
>  #define readb readb
>  static inline u8 readb(const volatile void __iomem *addr)
>  {
> -	return __raw_readb(addr);
> +	u8 val;
> +
> +	val = __raw_readb(addr);
> +	barrier();
> +
> +	return val;
>  }
>  #endif
>  
> @@ -118,7 +123,12 @@ static inline u8 readb(const volatile void __iomem *addr)
>  #define readw readw
>  static inline u16 readw(const volatile void __iomem *addr)
>  {
> -	return __le16_to_cpu(__raw_readw(addr));
> +	u16 val;
> +
> +	val = __le16_to_cpu(__raw_readw(addr));
> +	barrier();
> +
> +	return val;
>  }
>  #endif
>  
> @@ -126,7 +136,12 @@ static inline u16 readw(const volatile void __iomem *addr)
>  #define readl readl
>  static inline u32 readl(const volatile void __iomem *addr)
>  {
> -	return __le32_to_cpu(__raw_readl(addr));
> +	u32 val;
> +
> +	val = __le32_to_cpu(__raw_readl(addr));
> +	barrier();
> +
> +	return val;
>  }
>  #endif
>  
> @@ -135,7 +150,12 @@ static inline u32 readl(const volatile void __iomem *addr)
>  #define readq readq
>  static inline u64 readq(const volatile void __iomem *addr)
>  {
> -	return __le64_to_cpu(__raw_readq(addr));
> +	u64 val;
> +
> +	val = __le64_to_cpu(__raw_readq(addr));
> +	barrier();
> +
> +	return val;
>  }
>  #endif
>  #endif /* CONFIG_64BIT */
> -- 
> 2.7.4
> 
> --
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