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);
>
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