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Subjectperf events ring buffer memory barrier on powerpc
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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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