perf/core: Fix unlock balance in perf_init_event()

Commit:

  66d258c ("perf/core: Optimize perf_init_event()")

introduced an unlock imbalance in perf_init_event() where it calls
"goto again" and then only repeat rcu_read_unlock().

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: 66d258c ("perf/core: Optimize perf_init_event()")
Link: https://lkml.kernel.org/r/20191106052935.8352-1-cai@lca.pw
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -10307,7 +10307,6 @@ static struct pmu *perf_init_event(struct perf_event *event)
 			goto unlock;
 	}
 
-	rcu_read_lock();
 	/*
 	 * PERF_TYPE_HARDWARE and PERF_TYPE_HW_CACHE
 	 * are often aliases for PERF_TYPE_RAW.
@@ -10317,6 +10316,7 @@ static struct pmu *perf_init_event(struct perf_event *event)
 		type = PERF_TYPE_RAW;
 
 again:
+	rcu_read_lock();
 	pmu = idr_find(&pmu_idr, type);
 	rcu_read_unlock();
 	if (pmu) {