Re: perf events ring buffer memory barrier on powerpc

[Peter Zijlstra]

On Tue, Oct 29, 2013 at 11:21:31AM +0100, Peter Zijlstra wrote:
> On Mon, Oct 28, 2013 at 10:58:58PM +0200, Victor Kaplansky wrote:
> > Oleg Nesterov <oleg@xxxxxxxxxx> wrote on 10/28/2013 10:17:35 PM:
> > 
> > >       mb();   // XXXXXXXX: do we really need it? I think yes.
> > 
> > Oh, it is hard to argue with feelings. Also, it is easy to be on
> > conservative side and put the barrier here just in case.
> 
> I'll make it a full mb for now and too am curious to see the end of this
> discussion explaining things ;-)

That is, I've now got this queued:

---
Subject: perf: Fix perf ring buffer memory ordering
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Mon Oct 28 13:55:29 CET 2013

The PPC64 people noticed a missing memory barrier and crufty old
comments in the perf ring buffer code. So update all the comments and
add the missing barrier.

When the architecture implements local_t using atomic_long_t there
will be double barriers issued; but short of introducing more
conditional barrier primitives this is the best we can do.

Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Cc: michael@xxxxxxxxxxxxxx
Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Michael Neuling <mikey@xxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: anton@xxxxxxxxx
Cc: benh@xxxxxxxxxxxxxxxxxxx
Reported-by: Victor Kaplansky <victork@xxxxxxxxxx>
Tested-by: Victor Kaplansky <victork@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20131025173749.GG19466@xxxxxxxxxx
---
 include/uapi/linux/perf_event.h |   12 +++++++-----
 kernel/events/ring_buffer.c     |   31 +++++++++++++++++++++++++++----
 2 files changed, 34 insertions(+), 9 deletions(-)

Index: linux-2.6/include/uapi/linux/perf_event.h
===================================================================
--- linux-2.6.orig/include/uapi/linux/perf_event.h
+++ linux-2.6/include/uapi/linux/perf_event.h
@@ -479,13 +479,15 @@ struct perf_event_mmap_page {
 	/*
 	 * Control data for the mmap() data buffer.
 	 *
-	 * User-space reading the @data_head value should issue an rmb(), on
-	 * SMP capable platforms, after reading this value -- see
-	 * perf_event_wakeup().
+	 * User-space reading the @data_head value should issue an smp_rmb(),
+	 * after reading this value.
 	 *
 	 * When the mapping is PROT_WRITE the @data_tail value should be
-	 * written by userspace to reflect the last read data. In this case
-	 * the kernel will not over-write unread data.
+	 * written by userspace to reflect the last read data, after issueing
+	 * an smp_mb() to separate the data read from the ->data_tail store.
+	 * In this case the kernel will not over-write unread data.
+	 *
+	 * See perf_output_put_handle() for the data ordering.
 	 */
 	__u64   data_head;		/* head in the data section */
 	__u64	data_tail;		/* user-space written tail */
Index: linux-2.6/kernel/events/ring_buffer.c
===================================================================
--- linux-2.6.orig/kernel/events/ring_buffer.c
+++ linux-2.6/kernel/events/ring_buffer.c
@@ -87,10 +87,31 @@ static void perf_output_put_handle(struc
 		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.
+	 * Since the mmap() consumer (userspace) can run on a different CPU:
+	 *
+	 *   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
+	 *
+	 * 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.
+	 *
+	 * OTOH, 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
+	 * an RMB is sufficient since it separates two READs.
+	 *
+	 * See perf_output_begin().
 	 */
+	smp_wmb();
 	rb->user_page->data_head = head;
 
 	/*
@@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output
 		 * Userspace could choose to issue a mb() before updating the
 		 * tail pointer. So that all reads will be completed before the
 		 * write is issued.
+		 *
+		 * See perf_output_put_handle().
 		 */
 		tail = ACCESS_ONCE(rb->user_page->data_tail);
-		smp_rmb();
+		smp_mb();
 		offset = head = local_read(&rb->head);
 		head += size;
 		if (unlikely(!perf_output_space(rb, tail, offset, head)))
--
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