Re: [PATCH RFC 3/4] barriers: convert a control to a data dependency

[Jason Wang]

On 2019/1/3 äå4:57, Michael S. Tsirkin wrote:
> It's not uncommon to have two access two unrelated memory locations in a
> specific order.  At the moment one has to use a memory barrier for this.
>
> However, if the first access was a read and the second used an address
> depending on the first one we would have a data dependency and no
> barrier would be necessary.
>
> This adds a new interface: dependent_ptr_mb which does exactly this: it
> returns a pointer with a data dependency on the supplied value.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>   Documentation/memory-barriers.txt | 20 ++++++++++++++++++++
>   arch/alpha/include/asm/barrier.h  |  1 +
>   include/asm-generic/barrier.h     | 18 ++++++++++++++++++
>   include/linux/compiler.h          |  4 ++++
>   4 files changed, 43 insertions(+)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index c1d913944ad8..9dbaa2e1dbf6 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -691,6 +691,18 @@ case what's actually required is:
>   		p = READ_ONCE(b);
>   	}
>   
> +Alternatively, a control dependency can be converted to a data dependency,
> +e.g.:
> +
> +	q = READ_ONCE(a);
> +	if (q) {
> +		b = dependent_ptr_mb(b, q);
> +		p = READ_ONCE(b);
> +	}
> +
> +Note how the result of dependent_ptr_mb must be used with the following
> +accesses in order to have an effect.
> +
>   However, stores are not speculated.  This means that ordering -is- provided
>   for load-store control dependencies, as in the following example:
>   
> @@ -836,6 +848,12 @@ out-guess your code.  More generally, although READ_ONCE() does force
>   the compiler to actually emit code for a given load, it does not force
>   the compiler to use the results.
>   
> +Converting to a data dependency helps with this too:
> +
> +	q = READ_ONCE(a);
> +	b = dependent_ptr_mb(b, q);
> +	WRITE_ONCE(b, 1);
> +
>   In addition, control dependencies apply only to the then-clause and
>   else-clause of the if-statement in question.  In particular, it does
>   not necessarily apply to code following the if-statement:
> @@ -875,6 +893,8 @@ to the CPU containing it.  See the section on "Multicopy atomicity"
>   for more information.
>   
>   
> +
> +
>   In summary:
>   
>     (*) Control dependencies can order prior loads against later stores.
> diff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h
> index 92ec486a4f9e..b4934e8c551b 100644
> --- a/arch/alpha/include/asm/barrier.h
> +++ b/arch/alpha/include/asm/barrier.h
> @@ -59,6 +59,7 @@
>    * as Alpha, "y" could be set to 3 and "x" to 0.  Use rmb()
>    * in cases like this where there are no data dependencies.
>    */
> +#define ARCH_NEEDS_READ_BARRIER_DEPENDS 1
>   #define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
>   
>   #ifdef CONFIG_SMP
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index 2cafdbb9ae4c..fa2e2ef72b68 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -70,6 +70,24 @@
>   #define __smp_read_barrier_depends()	read_barrier_depends()
>   #endif
>   
> +#if defined(COMPILER_HAS_OPTIMIZER_HIDE_VAR) && \
> +	!defined(ARCH_NEEDS_READ_BARRIER_DEPENDS)
> +
> +#define dependent_ptr_mb(ptr, val) ({					\
> +	long dependent_ptr_mb_val = (long)(val);			\
> +	long dependent_ptr_mb_ptr = (long)(ptr) - dependent_ptr_mb_val;	\
> +									\
> +	BUILD_BUG_ON(sizeof(val) > sizeof(long));			\
> +	OPTIMIZER_HIDE_VAR(dependent_ptr_mb_val);			\
> +	(typeof(ptr))(dependent_ptr_mb_ptr + dependent_ptr_mb_val);	\
> +})
> +
> +#else
> +
> +#define dependent_ptr_mb(ptr, val) ({ mb(); (ptr); })


So for the example of patch 4, we'd better fall back to rmb() or need a 
dependent_ptr_rmb()?

Thanks


> +
> +#endif
> +
>   #ifdef CONFIG_SMP
>   
>   #ifndef smp_mb
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 6601d39e8c48..f599c30f1b28 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -152,9 +152,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>   #endif
>   
>   #ifndef OPTIMIZER_HIDE_VAR
> +
>   /* Make the optimizer believe the variable can be manipulated arbitrarily. */
>   #define OPTIMIZER_HIDE_VAR(var)						\
>   	__asm__ ("" : "=rm" (var) : "0" (var))
> +
> +#define COMPILER_HAS_OPTIMIZER_HIDE_VAR 1
> +
>   #endif
>   
>   /* Not-quite-unique ID. */