Re: [PATCH 1/1] use documented cmpxchg api

[Peter Hurley]

On 06/05/2014 09:03 PM, Pranith Kumar wrote:
> On 06/05/2014 08:34 PM, Peter Hurley wrote:
> > On 06/05/2014 06:19 PM, Pranith Kumar wrote:
> > > use the documented atomic_cmpxchg instead of __cmpxchg64
> > >
> > > This kills the last user of said API in drivers code.
> > >
> > >
> > > Signed-off-by: Pranith Kumar <bobby.prani@xxxxxxxxx>
> > > ---
> > >    drivers/iommu/intel-iommu.c |    2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> > > index 6bb3277..270113f 100644
> > > --- a/drivers/iommu/intel-iommu.c
> > > +++ b/drivers/iommu/intel-iommu.c
> > > @@ -293,7 +293,7 @@ static inline u64 dma_pte_addr(struct dma_pte *pte)
> > >        return pte->val & VTD_PAGE_MASK;
> > >    #else
> > >        /* Must have a full atomic 64-bit read */
> > > -    return  __cmpxchg64(&pte->val, 0ULL, 0ULL) & VTD_PAGE_MASK;
> > > +    return  atomic_cmpxchg(&pte->val, 0ULL, 0ULL) & VTD_PAGE_MASK;
> >
> > This is not equivalent.
> >
> > The __cmpxchg64() is specifically being used in this case
> > when !CONFIG_64BIT for a full 64-bit RMW, whereas atomic_cmpxchg()
> > uses ints which would be 32-bit RMW.
>
>
> You are right! The previous patch is wrong in that atomic_cmpxchg needs an atomic_t argument.
> cmpxchg() handles the size internally using typeof() to figure out the width of RMW.
> There is also an explicit cmpxchg64() which does the same thing.


This won't work either, and should generate a compile or link error
on CONFIG_X86_32.

The auto-sizing done by __cmpxchg() does not substitute the cmpxchg8b
instruction for the cmpxchg instruction. __X86_CASE_Q is defined as -1 for
CONFIG_X86_32 so the 8-byte size variant is dead-code, and calls
__cmpxchg_wrong_size() instead.


> Fixed with the following v2:
>
> use the documented cmpxchg instead of __cmpxchg64
>
> This kills the last user of said API in drivers code.
>
> Signed-off-by: Pranith Kumar <bobby.prani@xxxxxxxxx>
> ---
>   drivers/iommu/intel-iommu.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> index 6bb3277..270113f 100644
> --- a/drivers/iommu/intel-iommu.c
> +++ b/drivers/iommu/intel-iommu.c
> @@ -293,7 +293,7 @@ static inline u64 dma_pte_addr(struct dma_pte *pte)
>   	return pte->val & VTD_PAGE_MASK;
>   #else
>   	/* Must have a full atomic 64-bit read */
> -	return  __cmpxchg64(&pte->val, 0ULL, 0ULL) & VTD_PAGE_MASK;
> +	return  cmpxchg(&pte->val, 0ULL, 0ULL) & VTD_PAGE_MASK;
>   #endif
>   }


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/