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    SubjectRe: [RFC] Control dependencies
    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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