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

Mark Rutland
2018-04-03T10:49:25Z

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@codeaurora.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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