perf: Fix perf_pmu_migrate_context
While auditing the list_entry usage due to a trinity bug I found that
perf_pmu_migrate_context violates the rules for
perf_event::event_entry.
The problem is that perf_event::event_entry is a RCU list element, and
hence we must wait for a full RCU grace period before re-using the
element after deletion.
Therefore the usage in perf_pmu_migrate_context() which re-uses the
entry immediately is broken. For now introduce another list_head into
perf_event for this specific usage.
This doesn't actually fix the trinity report because that never goes
through this code.
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-mkj72lxagw1z8fvjm648iznw@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 866e85c..c8ba627 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -294,9 +294,31 @@
*/
struct perf_event {
#ifdef CONFIG_PERF_EVENTS
- struct list_head group_entry;
+ /*
+ * entry onto perf_event_context::event_list;
+ * modifications require ctx->lock
+ * RCU safe iterations.
+ */
struct list_head event_entry;
+
+ /*
+ * XXX: group_entry and sibling_list should be mutually exclusive;
+ * either you're a sibling on a group, or you're the group leader.
+ * Rework the code to always use the same list element.
+ *
+ * Locked for modification by both ctx->mutex and ctx->lock; holding
+ * either sufficies for read.
+ */
+ struct list_head group_entry;
struct list_head sibling_list;
+
+ /*
+ * We need storage to track the entries in perf_pmu_migrate_context; we
+ * cannot use the event_entry because of RCU and we want to keep the
+ * group in tact which avoids us using the other two entries.
+ */
+ struct list_head migrate_entry;
+
struct hlist_node hlist_entry;
int nr_siblings;
int group_flags;