Re: [PATCHv2 02/10] kernel: Provide READ_ONCE and ASSIGN_ONCE

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Nov 25, 2014 at 01:38:29PM +0100, Christian Borntraeger wrote:
> ACCESS_ONCE does not work reliably on non-scalar types. For
> example gcc 4.6 and 4.7 might remove the volatile tag for such
> accesses during the SRA (scalar replacement of aggregates) step
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)
> 
> Let's provide READ_ONCE/ASSIGN_ONCE that will do all accesses via
> scalar types.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  include/linux/compiler.h | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index d5ad7b1..0ff01f2 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -186,6 +186,40 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>  # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)
>  #endif
> 
> +#include <linux/types.h>
> +
> +static __always_inline void __assign_once_size(volatile void *p, void *res, int size)
> +{
> +	switch (size) {
> +	case 1: *(volatile u8 *)p = *(u8 *)res; break;
> +	case 2: *(volatile u16 *)p = *(u16 *)res; break;
> +	case 4: *(volatile u32 *)p = *(u32 *)res; break;
> +#ifdef CONFIG_64BIT
> +	case 8: *(volatile u64 *)p = *(u64 *)res; break;

We really need something like this to catch invalid sizes:

	default: invoke_nonexistent_function();

Of course, a BUILD_BUG_ON() would give a nicer error message.

Without this, in my testing, the following compiles without error, generating
no code:

	struct foo {
		int field[10];
	} f, f1;

	f1 = READ_ONCE(f);

There is probably some better way to do this.

> +#endif
> +       }
> +}
> +
> +#define ASSIGN_ONCE(val, p) \
> +      ({ typeof(p) __val; __val = val; __assign_once_size(&p, &__val, sizeof(__val)); __val; })
> +
> +
> +static __always_inline void __read_once_size(volatile void *p, void *res, int size)
> +{
> +	switch (size) {
> +	case 1: *(u8 *)res = *(volatile u8 *)p; break;
> +	case 2: *(u16 *)res = *(volatile u16 *)p; break;
> +	case 4: *(u32 *)res = *(volatile u32 *)p; break;
> +#ifdef CONFIG_64BIT
> +	case 8: *(u64 *)res = *(volatile u64 *)p; break;
> +#endif

Ditto here.

							Thanx, Paul

> +       }
> +}
> +
> +#define READ_ONCE(p) \
> +      ({ typeof(p) __val; __read_once_size(&p, &__val, sizeof(__val)); __val; })
> +
> +
>  #endif /* __KERNEL__ */
> 
>  #endif /* __ASSEMBLY__ */
> -- 
> 1.9.3
>