Re: [PATCH v10 9/9] arm64: armv8_deprecated: apply FEAT_LSUI for swpX emulation.

[Yeoreum Yun <yeoreum.yun@arm.com>]

[...]

> +static __always_inline int
> +__lsui_user_swpb_asm(unsigned int *data, unsigned int addr)
> +{
> +	unsigned char idx;
> +	int err;
> +	unsigned int addr_al;
> +	union {
> +		unsigned int var;
> +		unsigned char raw[4];
> +	} data_al;
> +
> +	idx = addr & (sizeof(unsigned int) - 1);
> +	addr_al = ALIGN_DOWN(addr, sizeof(unsigned int));
> +
> +	if (get_user(data_al.var, (unsigned int *)(unsigned long)addr_al))
> +		return -EFAULT;
> +
> +	data_al.raw[idx] = *data;
> +
> +	err = __lsui_user_swp_asm(&data_al.var, addr_al);

I found that using swpt for swpb can cause a race condition
when each CPU accesses its own dedicated byte memory region in 4 bytes.
So, unless there are any comments,
I’ll rework it with a cast in the next version.

> +	if (!err)
> +		*data = data_al.raw[idx];
> +
> +	return err;
> +}
> +#endif /* CONFIG_AS_HAS_LSUI */

Thanks.

--
Sincerely,
Yeoreum Yun