On Fri, Nov 22, 2013 at 12:59:41PM -0500, Peter Hurley wrote:
> On 11/22/2013 08:46 AM, Peter Zijlstra wrote:
> >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
>
>
> I wasn't subscribed to linux-arch so missed the smp_store_release()
> outcome, if there was one.
>
> Are (B) and (D) still slated for changing to STORE.rel semantics,
> aka smp_store_release()?
The earlier proposal would have A and C be smp_load_acquire() and B and
D be smp_store_release().
> I realize that, for the perf ring buffer, (D) is in userspace but
> I'm also interested in non-perf situations where (D) would be in the
> kernel.
So we're still debating the exact semantics of smp_store_release(), it
now looks like it needs a heavier memory barrier than previously
thought. In which case using it wouldn't make sense for me anymore.
Note that C and D are in userspace and not in any hot path (usually)
They're only issued once to read an entire buffer backlog at once, so I
don't really care about them all that much.
A and B otoh are in kernel space and are issued for every single event
written, so I'm interested to get them as cheaply as possible.
With this proposed patch, we remove a full barrier, with the earlier
smp_load_acquire() / smp_store_release() patches we would only
downgrade the full barrier to an acquire barrier, which is still more
than no barrier at all.
And now it looks like the smp_store_release() would actually upgrade the
wmb to a full barrier on some systems at least.
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