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

[Michael S. Tsirkin]

On Mon, Jan 07, 2019 at 11:58:23AM +0800, Jason Wang wrote:
> 
> 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

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.


> 
> > +
> > +#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. */