Re: [RFC] Control dependencies

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

On Fri, Nov 22, 2013 at 02:46:30PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 21, 2013 at 10:02:37AM -0800, Paul E. McKenney wrote:
> > > --- a/kernel/events/ring_buffer.c
> > > +++ b/kernel/events/ring_buffer.c
> > 
> > My patch does not cover this file.  Wouldn't hurt for them to be
> > separate.
> 
> Oh sure, but I wanted to present the RFC with at least one working
> example to illustrate why I even bother and to aid in discussion.
> 
> > > @@ -62,18 +62,18 @@ static void perf_output_put_handle(struc
> > >  	 *   kernel				user
> > >  	 *
> > >  	 *   READ ->data_tail			READ ->data_head
> > > -	 *   smp_mb()	(A)			smp_rmb()	(C)
> > > +	 *   barrier()	(A)			smp_rmb()	(C)
> > 
> > We need a conditional for this to work.  I know that the required
> > conditional is there in the code, but we need it explicitly in this
> > example as well.
> 
> Agreed, I skimped on that because I didn't quite know how to write that
> best.

Indeed, we still seem to be converging on that.

> How about the below version?

Much better!  Might even be correct.  ;-)

							Thanx, Paul

> ---
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -61,19 +61,20 @@ static void perf_output_put_handle(struc
>  	 *
>  	 *   kernel				user
>  	 *
> -	 *   READ ->data_tail			READ ->data_head
> -	 *   smp_mb()	(A)			smp_rmb()	(C)
> -	 *   WRITE $data			READ $data
> -	 *   smp_wmb()	(B)			smp_mb()	(D)
> -	 *   STORE ->data_head			WRITE ->data_tail
> +	 *   if (LOAD ->data_tail) {		LOAD ->data_head
> +	 *			(A)		smp_rmb()	(C)
> +	 *	STORE $data			LOAD $data
> +	 *	smp_wmb()	(B)		smp_mb()	(D)
> +	 *	STORE ->data_head		STORE ->data_tail
> +	 *   }
>  	 *
>  	 * Where A pairs with D, and B pairs with C.
>  	 *
> -	 * I don't think A needs to be a full barrier because we won't in fact
> -	 * write data until we see the store from userspace. So we simply don't
> -	 * issue the data WRITE until we observe it. Be conservative for now.
> +	 * In our case (A) is a control dependency that separates the load of
> +	 * the ->data_tail and the stores of $data. In case ->data_tail
> +	 * indicates there is no room in the buffer to store $data we do not.
>  	 *
> -	 * OTOH, D needs to be a full barrier since it separates the data READ
> +	 * D needs to be a full barrier since it separates the data READ
>  	 * from the tail WRITE.
>  	 *
>  	 * For B a WMB is sufficient since it separates two WRITEs, and for C
> @@ -81,7 +82,7 @@ static void perf_output_put_handle(struc
>  	 *
>  	 * See perf_output_begin().
>  	 */
> -	smp_wmb();
> +	smp_wmb(); /* B, matches C */
>  	rb->user_page->data_head = head;
> 
>  	/*
> @@ -144,17 +145,26 @@ int perf_output_begin(struct perf_output
>  		if (!rb->overwrite &&
>  		    unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
>  			goto fail;
> +
> +		/*
> +		 * The above forms a control dependency barrier separating the
> +		 * @tail load above from the data stores below. Since the @tail
> +		 * load is required to compute the branch to fail below.
> +		 *
> +		 * A, matches D; the full memory barrier userspace SHOULD issue
> +		 * after reading the data and before storing the new tail
> +		 * position.
> +		 *
> +		 * See perf_output_put_handle().
> +		 */
> +
>  		head += size;
>  	} while (local_cmpxchg(&rb->head, offset, head) != offset);
> 
>  	/*
> -	 * Separate the userpage->tail read from the data stores below.
> -	 * Matches the MB userspace SHOULD issue after reading the data
> -	 * and before storing the new tail position.
> -	 *
> -	 * See perf_output_put_handle().
> +	 * We rely on the implied barrier() by local_cmpxchg() to ensure
> +	 * none of the data stores below can be lifted up by the compiler.
>  	 */
> -	smp_mb();
> 
>  	if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
>  		local_add(rb->watermark, &rb->wakeup);
>