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.
How about the below version?
---
--- 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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