[PATCH] events: Annotate parent_ctx with __rcu

[Amol Grover <frextrite@gmail.com>]

parent_ctx is used under RCU context in kernel/events/core.c,
tell sparse about it aswell.

Fixes the following instances of sparse error:
kernel/events/core.c:3221:18: error: incompatible types in comparison
kernel/events/core.c:3222:23: error: incompatible types in comparison

This introduces the following two sparse errors:
kernel/events/core.c:3117:18: error: incompatible types in comparison
kernel/events/core.c:3121:30: error: incompatible types in comparison

Hence, use rcu_dereference() to access parent_ctx and silence
the newly introduced errors.

Signed-off-by: Amol Grover <frextrite@gmail.com>
---
 include/linux/perf_event.h |  2 +-
 kernel/events/core.c       | 11 ++++++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 6d4c22aee384..e18a7bdc6f98 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -810,7 +810,7 @@ struct perf_event_context {
 	 * These fields let us detect when two contexts have both
 	 * been cloned (inherited) from a common ancestor.
 	 */
-	struct perf_event_context	*parent_ctx;
+	struct perf_event_context __rcu	*parent_ctx;
 	u64				parent_gen;
 	u64				generation;
 	int				pin_count;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2173c23c25b4..2a8c5670b254 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
 static int context_equiv(struct perf_event_context *ctx1,
 			 struct perf_event_context *ctx2)
 {
+	struct perf_event_context *parent_ctx1, *parent_ctx2;
+
 	lockdep_assert_held(&ctx1->lock);
 	lockdep_assert_held(&ctx2->lock);
 
+	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
+	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
+
 	/* Pinning disables the swap optimization */
 	if (ctx1->pin_count || ctx2->pin_count)
 		return 0;
 
 	/* If ctx1 is the parent of ctx2 */
-	if (ctx1 == ctx2->parent_ctx && ctx1->generation == ctx2->parent_gen)
+	if (ctx1 == parent_ctx2 && ctx1->generation == ctx2->parent_gen)
 		return 1;
 
 	/* If ctx2 is the parent of ctx1 */
-	if (ctx1->parent_ctx == ctx2 && ctx1->parent_gen == ctx2->generation)
+	if (parent_ctx1 == ctx2 && ctx1->parent_gen == ctx2->generation)
 		return 1;
 
 	/*
 	 * If ctx1 and ctx2 have the same parent; we flatten the parent
 	 * hierarchy, see perf_event_init_context().
 	 */
-	if (ctx1->parent_ctx && ctx1->parent_ctx == ctx2->parent_ctx &&
+	if (ctx1->parent_ctx && parent_ctx1 == parent_ctx2 &&
 			ctx1->parent_gen == ctx2->parent_gen)
 		return 1;
 
-- 
2.24.1