Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

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

On Tue, Apr 28, 2015 at 04:32:35PM +0200, Peter Zijlstra wrote:
> On Sat, Apr 25, 2015 at 12:56:03PM -0700, Paul E. McKenney wrote:
> >     smp: Make control dependencies work on Alpha, improve documentation
> >     
> >     The current formulation of control dependencies fails on DEC Alpha, which
> >     does not respect dependencies of any kind unless an explicit memory is
> 
>  + barrier ?

Good catch, fixed!

> >     provided.  This means that the current fomulation of control dependencies
> >     fails on Alpha.  This commit therefore creates a READ_ONCE_CTRL() that
> >     has the same overhead on non-Alpha systems, but causes Alpha to produce
> >     the needed ordering.  This commit also applies READ_ONCE_CTRL() to the
> >     one known use of control dependencies.
> >     
> >     Use of READ_ONCE_CTRL() also has the beneficial effect of adding a bit
> >     of self-documentation to control dependencies.
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Applied, thank you!

							Thanx, Paul

> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 1b45e4a0519b..a57eacde2b84 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -264,6 +264,22 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
> >  #define WRITE_ONCE(x, val) \
> >  	({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; })
> >  
> > +/**
> > + * READ_ONCE_CTRL - Read a value heading a control dependency
> > + * @x: The value to be read, heading the control dependency
> > + *
> > + * Control dependencies are tricky.  See Documentation/memory-barriers.txt
> > + * for important information on how to use them.  Note that in many cases,
> > + * use of smp_load_acquire() will be much simpler.  Control dependencies
> > + * should be avoided except on the hottest of hotpaths.
> > + */
> > +#define READ_ONCE_CTRL(x) \
> > +({ \
> > +	typeof(x) __val = READ_ONCE(x); \
> > +	smp_read_barrier_depends(); /* Enforce control dependency. */ \
> > +	__val; \
> > +})
> > +
> >  #endif /* __KERNEL__ */
> >  
> >  #endif /* __ASSEMBLY__ */
> 
> We mostly try and align the \ for multi-line macros.
> 
> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > index eadb95ce7aac..67548d5de4cb 100644
> > --- a/kernel/events/ring_buffer.c
> > +++ b/kernel/events/ring_buffer.c
> > @@ -141,7 +141,7 @@ int perf_output_begin(struct perf_output_handle *handle,
> >  	perf_output_get_handle(handle);
> >  
> >  	do {
> > -		tail = ACCESS_ONCE(rb->user_page->data_tail);
> > +		tail = READ_ONCE_CTRL(rb->user_page->data_tail);
> >  		offset = head = local_read(&rb->head);
> >  		if (!rb->overwrite &&
> >  		    unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
> 
> Right. I could not remember any other current usage.
>