aboutsummaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorPeter Zijlstra <peterz@infradead.org>2013-10-28 13:55:29 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-11-20 12:27:47 -0800
commit0fb024c4377701319ae29af1b23b647885d7c808 (patch)
tree547d735b894f012fae9300beed0b83b7cf3af5a6
parent8430063608b2a95a75cdf3ddd3fbd7dac88c151f (diff)
downloadvexpress-lsk-0fb024c4377701319ae29af1b23b647885d7c808.tar.gz
perf: Fix perf ring buffer memory ordering
commit bf378d341e4873ed928dc3c636252e6895a21f50 upstream. 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. Reported-by: Victor Kaplansky <victork@il.ibm.com> Tested-by: Victor Kaplansky <victork@il.ibm.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Cc: michael@ellerman.id.au Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Cc: Michael Neuling <mikey@neuling.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: anton@samba.org Cc: benh@kernel.crashing.org Link: http://lkml.kernel.org/r/20131025173749.GG19466@laptop.lan Signed-off-by: Ingo Molnar <mingo@kernel.org> Cc: Michael Neuling <mikey@neuling.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--include/uapi/linux/perf_event.h12
-rw-r--r--kernel/events/ring_buffer.c31
2 files changed, 34 insertions, 9 deletions
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index fb104e51496..9e59950f55c 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -425,13 +425,15 @@ 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 */
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index cd55144270b..9c2ddfbf452 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -87,10 +87,31 @@ 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.
+ * 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_handle *handle,
* 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)))