Messages in this thread Patch in this message |  | | From | Michael Neuling <> | Subject | perf events ring buffer memory barrier on powerpc | Date | Wed, 23 Oct 2013 10:54:54 +1100 |
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Frederic,
In the perf ring buffer code we have this in perf_output_get_handle():
if (!local_dec_and_test(&rb->nest)) goto out;
/* * Publish the known good head. Rely on the full barrier implied * by atomic_dec_and_test() order the rb->head read and this * write. */ rb->user_page->data_head = head;
The comment says atomic_dec_and_test() but the code is local_dec_and_test().
On powerpc, local_dec_and_test() doesn't have a memory barrier but atomic_dec_and_test() does. Is the comment wrong, or is local_dec_and_test() suppose to imply a memory barrier too and we have it wrongly implemented in powerpc?
My guess is that local_dec_and_test() is correct but we to add an explicit memory barrier like below:
(Kudos to Victor Kaplansky for finding this)
Mikey
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index cd55144..95768c6 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -87,10 +87,10 @@ again: goto out; /* - * Publish the known good head. Rely on the full barrier implied - * by atomic_dec_and_test() order the rb->head read and this - * write. + * Publish the known good head. We need a memory barrier to order the + * order the rb->head read and this write. */ + smp_mb (); rb->user_page->data_head = head; /*
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