[PATCH RFC] perf: fix find_swevent_head() RCU lockdep splat

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

This commit guesses at the perf_cpu_context locking design and deploys
an rcu_dereference_check() accordingly.  The design appears to require
that a given CPU be accessing its own per_cpu_context or that it be
traversing under RCU protection.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>

 perf_event.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index a4fa381..002791c 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -4074,7 +4074,9 @@ find_swevent_head(struct perf_cpu_context *ctx, u64 type, u32 event_id)
 
 	hash = swevent_hash(type, event_id);
 
-	hlist = rcu_dereference(ctx->swevent_hlist);
+	hlist = rcu_dereference_check(ctx->swevent_hlist,
+				      rcu_read_lock_held() ||
+				      ctx == &__get_cpu_var(perf_cpu_context));
 	if (!hlist)
 		return NULL;