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

[Jason Wang <jasowang@redhat.com>]

On 2019/1/7 下午12:23, Michael S. Tsirkin wrote:
> On Mon, Jan 07, 2019 at 11:58:23AM +0800, Jason Wang wrote:
>> On 2019/1/3 上午4:57, 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@redhat.com>
>>> ---
>>>    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
> You mean for strongly ordered architectures like Intel?
> Yes, maybe it makes sense to have dependent_ptr_smp_rmb,
> dependent_ptr_dma_rmb and dependent_ptr_virt_rmb.
>
> mb variant is unused right now so I'll remove it.
>
>

Yes.

Thanks


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